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<div class="chapter" title="Chapter 2. SQL Language">
<div class="titlepage">
<div>
<div>
<h2 class="title">
<a name="sqlgeneral-chapt"></a>Chapter 2. SQL Language</h2>
</div>
<div>
<div class="authorgroup">
<div class="author">
<h3 class="author">
<span class="firstname">Fred</span> <span class="surname">Toussi</span>
</h3>
<div class="affiliation">
<span class="orgname">The HSQL Development Group<br>
</span>
</div>
</div>
</div>
</div>
<div>
<p class="releaseinfo">$Revision: 5701 $</p>
</div>
<div>
<div class="legalnotice" title="Legal Notice">
<a name="N10359"></a>
<p>Copyright 2002-2017 Fred Toussi. Permission is granted to
distribute this document without any alteration under the terms of the
HSQLDB license. Additional permission is granted to the HSQL Development
Group to distribute this document with or without alterations under the
terms of the HSQLDB license.</p>
</div>
</div>
<div>
<p class="pubdate">2017-04-09 14:14:09-0400</p>
</div>
</div>
</div>
<div class="toc">
<p>
<b>Table of Contents</b>
</p>
<dl>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_standards">Standards Support</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_data_tables">SQL Data and Tables</a></span>
</dt>
<dd>
<dl>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_temp_tables">Temporary Tables</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_persist_tables">Persistent Tables</a></span>
</dt>
</dl>
</dd>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_data_type_guide">Short Guide to Data Types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_types_ops">Data Types and Operations</a></span>
</dt>
<dd>
<dl>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_numeric_types">Numeric Types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_boolean_type">Boolean Type</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_char_types">Character String Types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_binary_types">Binary String Types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_bit_types">Bit String Types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_lob_data">Lob Data</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_java_objects">Storage and Handling of Java Objects</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_length_precision">Type Length, Precision and Scale</a></span>
</dt>
</dl>
</dd>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_datetime_types">Datetime types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_interval_typs">Interval Types</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_array">Arrays</a></span>
</dt>
<dd>
<dl>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_array_def">Array Definition</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_array_ref">Array Reference</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_array_ops">Array Operations</a></span>
</dt>
</dl>
</dd>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_index_speed">Indexes and Query Speed</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_query_opt">Query Processing and Optimisation</a></span>
</dt>
<dd>
<dl>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_indexes_cond">Indexes and Conditions</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_indexes_ops">Indexes and Operations</a></span>
</dt>
<dt>
<span class="section"><a href="sqlgeneral-chapt.html#sgc_indexes_order">Indexes and ORDER BY, OFFSET and LIMIT</a></span>
</dt>
</dl>
</dd>
</dl>
</div>
<div class="section" title="Standards Support">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_standards"></a>Standards Support</h2>
</div>
</div>
</div>
<p>HyperSQL 2.x supports the dialect of SQL defined by SQL standards
92, 1999, 2003, 2008 and 2011. This means where a feature of the standard
is supported, e.g. left outer join, the syntax is that specified by the
standard text. Almost all syntactic features of SQL-92 up to Advanced
Level are supported, as well as SQL:2011 core and many optional features
of this standard. Work is in progress for a formal declaration of
conformance.</p>
<p>At the time of this release, HyperSQL supports the widest range of
SQL standard features among all open source RDBMS.</p>
<p>Various chapters of this guide list the supported syntax. When
writing or converting existing SQL DDL (Data Definition Language), DML
(Data Manipulation Language) or DQL (Data Query Language) statements for
HSQLDB, you should consult the supported syntax and modify the statements
accordingly. Some statements written for older versions may have to be
modified.</p>
<p>Over 300 words are reserved by the standard and should not be used
as table or column names. For example, the word POSITION is reserved as it
is a function defined by the Standards with a similar role as
<code class="methodname">String.indexOf()</code> in Java. HyperSQL does not
currently prevent you from using a reserved word if it does not support
its use or can distinguish it. For example CUBE is a reserved words that
is not currently supported by HyperSQL and is allowed as a table or column
name. You should avoid using such names as future versions of HyperSQL are
likely to support the reserved words and may reject your table definitions
or queries. The full list of SQL reserved words is in the appendix <a class="link" href="lists-app.html" title="Appendix A. Lists of Keywords">Lists of Keywords</a> .</p>
<p>There are several user-defined properties to control the strict
application of the SQL Standard in different areas.</p>
<p>If you have to use a reserved keyword as the name of a database
object, you can enclose it in double quotes.</p>
<p>HyperSQL also supports enhancements with keywords and expressions
that are not part of the SQL standard. Expressions such as <code class="literal">SELECT
TOP 5 FROM ..</code>, <code class="literal">SELECT LIMIT 0 10 FROM ...</code> or
<code class="literal">DROP TABLE mytable IF EXISTS</code> are among such
constructs.</p>
<p>Many print books cover SQL Standard syntax and can be
consulted.</p>
<p>In HyperSQL version 2, all features of JDBC4 that apply to the
capabilities of HSQLDB are fully supported. The relevant JDBC classes are
thoroughly documented with additional clarifications and HyperSQL specific
comments. See the <a class="link" href="filelinks-app.html#javadoc-link">JavaDoc</a> for the
<code class="classname">org.hsqldb.jdbc.*</code> classes.</p>
</div>
<div class="section" title="SQL Data and Tables">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_data_tables"></a>SQL Data and Tables</h2>
</div>
</div>
</div>
<p>In an SQL system, all significant data is stored in tables and
sequence generators. Therefore, the first step in creating a database is
defining the tables and their columns. The SQL standard supports temporary
tables, which are for temporary data, and permanent base tables, which are
for persistent data.</p>
<div class="section" title="Temporary Tables">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_temp_tables"></a>Temporary Tables</h3>
</div>
</div>
</div>
<p>Data in TEMPORARY tables is not saved and lasts only for the
lifetime of the session. The contents of each TEMP table is visible only
from the session that is used to populate it.</p>
<p>HyperSQL supports two types of temporary tables.</p>
<p>The <code class="literal">GLOBAL TEMPORARY</code> type is a schema object.
It is created with the <code class="literal">CREATE GLOBAL TEMPORARY TABLE</code>
statement. The definition of the table persists, and each session has
access to the table. But each session sees its own copy of the table,
which is empty at the beginning of the session.</p>
<p>The <code class="literal">LOCAL TEMPORARY</code> type is not a schema
object. It is created with the <code class="literal">DECLARE LOCAL TEMPORARY
TABLE</code> statement. The table definition lasts only for the
duration of the session and is not persisted in the database. The table
can be declared in the middle of a transaction without committing the
transaction. If a schema name is needed to reference these tables in a
given SQL statement, the pseudo schema names <code class="literal">MODULE</code>
or <code class="literal">SESSION</code> can be used.</p>
<p>When the session commits, the contents of all temporary tables are
cleared by default. If the table definition statements includes ON
COMMIT PRESERVE ROWS, then the contents are kept when a commit takes
place.</p>
<p>The rows in temporary tables are stored in memory by default. If
the <code class="literal">hsqldb.result_max_memory_rows</code> property has been
set or the <code class="literal">SET SESSION RESULT MEMORY ROWS <row
count></code> has been specified, tables with row count above the
setting are stored on disk.</p>
</div>
<div class="section" title="Persistent Tables">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_persist_tables"></a>Persistent Tables</h3>
</div>
</div>
</div>
<p>HyperSQL supports the Standard definition of persistent base
table, but defines three types according to the way the data is stored.
These are MEMORY tables, CACHED tables and TEXT tables.</p>
<p>Memory tables are the default type when the CREATE TABLE command
is used. Their data is held entirely in memory but any change to their
structure or contents is written to the <code class="filename">*.log</code> and
<code class="filename">*.script</code> files. The <code class="filename">*.script</code>
file and the <code class="filename">*.log</code> file are read the next time the
database is opened, and the MEMORY tables are recreated with all their
contents. So unlike TEMPORARY tables, MEMORY tables are persistent. When
the database is opened, all the data for the memory tables is read and
inserted. This process may take a long time if the database is larger
than tens of megabytes. When the database is shutdown, all the data is
saved. This can also take a long time.</p>
<p>CACHED tables are created with the CREATE CACHED TABLE command.
Only part of their data or indexes is held in memory, allowing large
tables that would otherwise take up to several hundred megabytes of
memory. Another advantage of cached tables is that the database engine
takes less time to start up when a cached table is used for large
amounts of data. The disadvantage of cached tables is a reduction in
speed. Do not use cached tables if your data set is relatively small. In
an application with some small tables and some large ones, it is better
to use the default, MEMORY mode for the small tables.</p>
<p>TEXT tables use a CSV (Comma Separated Value) or other delimited
text file as the source of their data. You can specify an existing CSV
file, such as a dump from another database or program, as the source of
a TEXT table. Alternatively, you can specify an empty file to be filled
with data by the database engine. TEXT tables are efficient in memory
usage as they cache only part of the text data and all of the indexes.
The Text table data source can always be reassigned to a different file
if necessary. The commands are needed to set up a TEXT table as detailed
in the <a class="link" href="texttables-chapt.html" title="Chapter 5. Text Tables">Text Tables</a> chapter.</p>
<p>With all-in-memory databases, both MEMORY table and CACHED table
declarations are treated as declarations for non-persistent memory
tables. In the latest versions of HyperSQL, TEXT table declarations are
allowed in all-in-memory databases.</p>
<p>The default type of tables resulting from future CREATE TABLE
statements can be specified with the SQL command:</p>
<pre class="programlisting"> SET DATABASE DEFAULT TABLE TYPE { CACHED | MEMORY };</pre>
<p>The
type of an existing table can be changed with the SQL command:</p>
<pre class="programlisting"> SET TABLE <table name> TYPE { CACHED | MEMORY };</pre>
<p>SQL
statements access different types of tables uniformly. No change to
statements is needed to access different types of table.</p>
</div>
</div>
<div class="section" title="Short Guide to Data Types">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_data_type_guide"></a>Short Guide to Data Types</h2>
</div>
</div>
</div>
<p>Most other RDBMS do not conform to the SQL Standard in all areas,
but they are gradually moving towards Standard conformance. When switching
from another SQL dialect, the following should be considered:</p>
<div class="itemizedlist">
<ul class="itemizedlist" type="disc">
<li class="listitem">
<p>Numeric types TINYINT, SMALLINT, INTEGER and BIGINT are types
with fixed binary precision. These types are more efficient to store
and retrieve. NUMERIC and DECIMAL are types with user-defined decimal
precision. They can be used with zero scale to store very large
integers, or with a non-zero scale to store decimal fractions. The
DOUBLE type is a 64 bit, approximate floating point types. HyperSQL
even allows you to store infinity in this type.</p>
</li>
<li class="listitem">
<p>The BOOLEAN type is for logical values and can hold TRUE, FALSE
or UNKNOWN. Although HyperSQL allows you to use one and zero in
assignment or comparison, you should use the standard values for this
type.</p>
</li>
<li class="listitem">
<p>Character string types are CHAR(L), VARCHAR(L) and CLOB. CHAR is
for fixed width strings and any string that is assigned to this type
is padded with spaces at the end. Do not use this type for general
storage of strings. If you use CHAR without the length L, then it is
interpreted as a single character string. Use VARCHAR(L) for general
strings. There are only memory limits and performance implications for
the maximum length of VARCHAR(L). If the strings are larger than a few
kilobytes, consider using CLOB. The CLOB types is for very large
strings. Do not use this type for short strings as there are
performance implications. The CLOB type is a better choice for the
storage of long strings. By default LONGVARCHAR is a synonym for a
long VARCHAR and can be used without specifying the size. You can set
LONGVARCHAR to map to CLOB, with the
<code class="literal">sql.longvar_is_lob</code> connection property or the SET
DATABASE SQL LONGVAR IS LOB TRUE statement.</p>
</li>
<li class="listitem">
<p>Binary string types are BINARY(L), VARBINARY(L) and BLOB. Do not
use BINARY(L) unless you are storing keys such as UUID. This type pads
short binary strings with zero bytes. BINARY without the length L
means a single byte. Use VARBINARY(L) for general binary strings, and
BLOB for large binary objects. You should apply the same
considerations as with the character string types. By default
LONGVARBINARY is a synonym for a long VARCHAR and can be used without
specifying the size. You can set LONGVARBINARY to map to BLOB, with
the <code class="literal">sql.longvar_is_lob</code> connection property or the
SET DATABASE SQL LONGVAR IS LOB TRUE statement.</p>
</li>
<li class="listitem">
<p>The BIT(L) and BITVARYING(L) types are for bit maps. Do not use
them for other types of data. BIT without the length L argument means
a single bit and is sometimes used as a logical type. Use BOOLEAN
instead of this type.</p>
</li>
<li class="listitem">
<p>The UUID type is for UUID (also called GUID) values. The value
is stored as BINARY. UUID character strings, as well as BINARY
strings, can be used to insert or to compare.</p>
</li>
<li class="listitem">
<p>The datetime types DATE, TIME and TIMESTAMP, together with their
WITH TIME ZONE variations are available. Read the details in this
chapter on how to use these types.</p>
</li>
<li class="listitem">
<p>The INTERVAL type is very powerful when used together with the
datetime types. This is very easy to use, but is supported mainly by
"big iron" database systems. Note that functions that add days or
months to datetime values are not really a substitute for the INTERVAL
type. Expressions such as <code class="literal">(datecol - 7 DAY) >
CURRENT_DATE</code> are optimised to use indexes when it is
possible, while the equivalent function calls are not
optimised.</p>
</li>
<li class="listitem">
<p>The OTHER type is for storage of Java objects. If your objects
are large, serialize them in your application and store them as BLOB
in the database.</p>
</li>
<li class="listitem">
<p>The ARRAY type supports all base types except LOB and OTHER
types. ARRAY data objects are held in memory while being processed. It
is therefore not recommended to store more than about a thousand
objects in an ARRAY in normal operations with disk based databases.
For specialised applications, use ARRAY with as many elements as your
memory allocation can support.</p>
</li>
</ul>
</div>
<p>HyperSQL 2.3 has several compatibility modes which allow the type
names that are used by other RDBMS to be accepted and translated into the
closest SQL Standard type. For example the type TEXT, supported by MySQL
and PostgreSQL is translated in these compatibility modes.</p>
</div>
<div class="section" title="Data Types and Operations">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_types_ops"></a>Data Types and Operations</h2>
</div>
</div>
</div>
<p>HyperSQL supports all the types defined by SQL-92, plus BOOLEAN,
BINARY and LOB types that were later added to the SQL Standard. It also
supports the non-standard OTHER type to store serializable Java
objects.</p>
<p>SQL is a strongly typed language. All data stored in specific
columns of tables and other objects (such as sequence generators) have
specific types. Each data item conforms to the type limits such as
precision and scale for the column. It also conforms to any additional
integrity constraints that are defined as CHECK constraints in domains or
tables. Types can be explicitly converted using the CAST expression, but
in most expressions they are converted automatically.</p>
<p>Data is returned to the user (or the application program) as a
result of executing SQL statements such as query expressions or function
calls. All statements are compiled prior to execution and the return type
of the data is known after compilation and before execution. Therefore,
once a statement is prepared, the data type of each column of the returned
result is known, including any precision or scale property. The type does
not change when the same query that returned one row, returns many rows as
a result of adding more data to the tables.</p>
<p>Some SQL functions used within SQL statements are polymorphic, but
the exact type of the argument and the return value is determined at
compile time.</p>
<p>When a statement is prepared, using a JDBC PreparedStatement object,
it is compiled by the engine and the type of the columns of its ResultSet
and / or its parameters are accessible through the methods of
PreparedStatement.</p>
<div class="section" title="Numeric Types">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_numeric_types"></a>Numeric Types</h3>
</div>
</div>
</div>
<a name="N10421" class="indexterm"></a>
<p>TINYINT, SMALLINT, INTEGER, BIGINT, NUMERIC and DECIMAL (without a
decimal point) are the supported integral types. They correspond
respectively to <code class="classname">byte</code>,
<code class="classname">short</code>, <code class="classname">int</code>,
<code class="classname">long</code>, <code class="classname">BigDecimal</code> and
<code class="classname">BigDecimal</code> Java types in the range of values that
they can represent (NUMERIC and DECIMAL are equivalent). The type
TINYINT is an HSQLDB extension to the SQL Standard, while the others
conform to the Standard definition. The SQL type dictates the maximum
and minimum values that can be held in a field of each type. For example
the value range for TINYINT is -128 to +127. The bit precision of
TINYINT, SMALLINT, INTEGER and BIGINT is respectively 8, 16, 32 and 64.
For NUMERIC and DECIMAL, decimal precision is used.</p>
<p>DECIMAL and NUMERIC with decimal fractions are mapped to
<code class="classname">java.math.BigDecimal</code> and can have very large
numbers of digits. In HyperSQL the two types are equivalent. These
types, together with integral types, are called exact numeric
types.</p>
<p>In HyperSQL, REAL, FLOAT, DOUBLE are equivalent and all mapped to
<code class="classname">double</code> in Java. These types are defined by the
SQL Standard as approximate numeric types. The bit-precision of all
these types is 64 bits.</p>
<p>The decimal precision and scale of NUMERIC and DECIMAL types can
be optionally defined. For example, DECIMAL(10,2) means maximum total
number of digits is 10 and there are always 2 digits after the decimal
point, while DECIMAL(10) means 10 digits without a decimal point. The
bit-precision of FLOAT can be defined but it is ignored and the default
bit-precision of 64 is used. The default precision of NUMERIC and
DECIMAL (when not defined) is 100.</p>
<p>Note: If a database has been set to ignore type precision limits
with the SET DATABASE SQL SIZE FALSE command, then a type definition of
DECIMAL with no precision and scale is treated as DECIMAL(100,10). In
normal operation, it is treated as DECIMAL(100).</p>
<p>
<span class="bold"><strong>Integral Types</strong></span>
</p>
<p>In expressions, TINYINT, SMALLINT, INTEGER, BIGINT, NUMERIC and
DECIMAL (without a decimal point) can be freely combined and no data
narrowing takes place. The resulting value is of a type that can support
all possible values.</p>
<p>If the SELECT statement refers to a simple column or function,
then the return type is the type corresponding to the column or the
return type of the function. For example:</p>
<div class="informalexample">
<pre class="programlisting"> CREATE TABLE t(a INTEGER, b BIGINT);
SELECT MAX(a), MAX(b) FROM t;</pre>
</div>
<p>will return a <code class="classname">ResultSet</code> where the type of
the first column is <code class="classname">java.lang.Integer</code> and the
second column is <code class="classname">java.lang.Long</code>. However,</p>
<div class="informalexample">
<pre class="programlisting"> SELECT MAX(a) + 1, MAX(b) + 1 FROM t;</pre>
</div>
<p>will return <code class="classname">java.lang.Long</code> and
<code class="classname">BigDecimal</code> values, generated as a result of
uniform type promotion for all the return values. Note that type
promotion to <code class="classname">BigDecimal</code> ensures the correct value
is returned if <code class="literal">MAX(b)</code> evaluates to
<code class="literal">Long.MAX_VALUE</code>.</p>
<p>There is no built-in limit on the size of intermediate integral
values in expressions. As a result, you should check for the type of the
<code class="classname">ResultSet</code> column and choose an appropriate
<code class="methodname">getXXXX()</code> method to retrieve it. Alternatively,
you can use the <code class="methodname">getObject()</code> method, then cast
the result to <code class="classname">java.lang.Number</code> and use the
<code class="methodname">intValue()</code> or
<code class="methodname">longValue()</code> methods on the result.</p>
<p>When the result of an expression is stored in a column of a
database table, it has to fit in the target column, otherwise an error
is returned. For example when <code class="literal">1234567890123456789012 /
12345687901234567890</code> is evaluated, the result can be stored in
any integral type column, even a TINYINT column, as it is a small
value.</p>
<p>In SQL Statements, an integer literal is treated as INTEGER,
unless its value does not fit. In this case it is treated as BIGINT or
DECIMAL, depending on the value.</p>
<p>Depending on the types of the operands, the result of the
operation is returned in a JDBC <code class="classname">ResultSet</code> in any
of the related Java types: <code class="classname">Integer</code>,
<code class="classname">Long</code> or <code class="classname">BigDecimal</code>. The
<code class="methodname">ResultSet.getXXXX()</code> methods can be used to
retrieve the values so long as the returned value can be represented by
the resulting type. This type is deterministically based on the query,
not on the actual rows returned.</p>
<p>
<span class="bold"><strong>Other Numeric Types</strong></span>
</p>
<p>In SQL statements, number literals with a decimal point are
treated as DECIMAL unless they are written with an exponent. Thus
<code class="literal">0.2</code> is considered a DECIMAL value but
<code class="literal">0.2E0</code> is considered a DOUBLE value.</p>
<p>When an approximate numeric type, REAL, FLOAT or DOUBLE (all
synonymous) is part of an expression involving different numeric types,
the type of the result is DOUBLE. DECIMAL values can be converted to
DOUBLE unless they are beyond the <code class="literal">Double.MIN_VALUE -
Double.MAX_VALUE</code> range. For example, A * B, A / B, A + B, etc.
will return a DOUBLE value if either A or B is a DOUBLE.</p>
<p>Otherwise, when no DOUBLE value exists, if a DECIMAL or NUMERIC
value is part an expression, the type of the result is DECIMAL or
NUMERIC. Similar to integral values, when the result of an expression is
assigned to a table column, the value has to fit in the target column,
otherwise an error is returned. This means a small, 4 digit value of
DECIMAL type can be assigned to a column of SMALLINT or INTEGER, but a
value with 15 digits cannot.</p>
<p>When a DECIMAL value is multiplied by a DECIMAL or integral type,
the resulting scale is the sum of the scales of the two terms. When they
are divided, the result is a value with a scale (number of digits to the
right of the decimal point) equal to the larger of the scales of the two
terms. The precision for both operations is calculated (usually
increased) to allow all possible results.</p>
<p>The distinction between DOUBLE and DECIMAL is important when a
division takes place. For example, <code class="literal">10.0/8.0</code> (DECIMAL)
equals <code class="literal">1.2</code> but <code class="literal">10.0E0/8.0E0</code>
(DOUBLE) equals <code class="literal">1.25</code>. Without division operations,
DECIMAL values represent exact arithmetic.</p>
<p>REAL, FLOAT and DOUBLE values are all stored in the database as
<code class="classname">java.lang.Double</code> objects. Special values such as
NaN and +-Infinity are also stored and supported. These values can be
submitted to the database via JDBC
<code class="classname">PreparedStatement</code> methods and are returned in
<code class="classname">ResultSet</code> objects. In order to allow division by
zero of DOUBLE values in SQL statements (which returns NaN or
+-Infinity) you should set the property hsqldb.double_nan as false (SET
DATABASE SQL DOUBLE NAN FALSE). The double values can be retrieved from
a <code class="classname">ResultSet</code> in the required type so long as they
can be represented. For setting the values, when
<code class="methodname">PreparedStatement.setDouble()</code> or
<code class="methodname">setFloat()</code> is used, the value is treated as a
DOUBLE automatically.</p>
<p>In short,</p>
<p>
<code class="literal"><numeric type> ::= <exact numeric type> |
<approximate numeric type></code>
</p>
<p>
<code class="literal"><exact numeric type> ::= NUMERIC [ <left
paren> <precision> [ <comma> <scale> ] <right
paren> ] | { DECIMAL | DEC } [ <left paren> <precision> [
<comma> <scale> ] <right paren> ] | SMALLINT | INTEGER
| INT | BIGINT</code>
</p>
<p>
<code class="literal"><approximate numeric type> ::= FLOAT [ <left
paren> <precision> <right paren> ] | REAL | DOUBLE
PRECISION</code>
</p>
<p>
<code class="literal"><precision> ::= <unsigned
integer></code>
</p>
<p>
<code class="literal"><scale> ::= <unsigned
integer></code>
</p>
</div>
<div class="section" title="Boolean Type">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_boolean_type"></a>Boolean Type</h3>
</div>
</div>
</div>
<a name="N104EA" class="indexterm"></a>
<p>The BOOLEAN type conforms to the SQL Standard and represents the
values <code class="literal">TRUE</code>, <code class="literal">FALSE</code> and
<code class="literal">UNKNOWN</code>. This type of column can be initialised with
Java boolean values, or with <code class="literal">NULL</code> for the
<code class="literal">UNKNOWN</code> value.</p>
<p>The three-value logic is sometimes misunderstood. For example, x
IN (1, 2, NULL) does not return true if x is NULL.</p>
<p>In previous versions of HyperSQL, BIT was simply an alias for
BOOLEAN. In version 2, BIT is a single-bit bit map.</p>
<p>
<code class="literal"><boolean type> ::= BOOLEAN</code>
</p>
<p>The SQL Standard does not support type conversion to BOOLEAN apart
from character strings that consists of boolean literals. Because the
BOOLEAN type is relatively new to the Standard, several database
products used other types to represent boolean values. For improved
compatibility, HyperSQL allows some type conversions to boolean.</p>
<p>Values of BIT and BIT VARYING types with length 1 can be converted
to BOOLEAN. If the bit is set, the result of conversion is the TRUE
value, otherwise it is FALSE.</p>
<p>Values of TINYINT, SMALLINT, INTEGER and BIGINT types can be
converted to BOOLEAN. If the value is zero, the result is the FALSE
value, otherwise it is TRUE.</p>
</div>
<div class="section" title="Character String Types">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_char_types"></a>Character String Types</h3>
</div>
</div>
</div>
<a name="N10511" class="indexterm"></a>
<p>The CHARACTER, CHARACTER VARYING and CLOB types are the SQL
Standard character string types. CHAR, VARCHAR and CHARACTER LARGE
OBJECT are synonyms for these types. HyperSQL also supports LONGVARCHAR
as a synonym for VARCHAR. If LONGVARCHAR is used without a length, then
a length of 16M is assigned. You can set LONGVARCHAR to map to CLOB,
with the <code class="literal">sql.longvar_is_lob</code> connection property or
the SET DATABASE SQL LONGVAR IS LOB TRUE statement..</p>
<p>HyperSQL's default character set is Unicode, therefore all
possible character strings can be represented by these types.</p>
<p>The SQL Standard behaviour of the CHARACTER type is a remnant of
legacy systems in which character strings are padded with spaces to fill
a fixed width. These spaces are sometimes significant while in other
cases they are silently discarded. It would be best to avoid the
CHARACTER type altogether. With the rest of the types, the strings are
not padded when assigned to columns or variables of the given type. The
trailing spaces are still considered discardable for all character
types. Therefore if a string with trailing spaces is too long to assign
to a column or variable of a given length, the spaces beyond the type
length are discarded and the assignment succeeds (provided all the
characters beyond the type length are spaces).</p>
<p>The VARCHAR and CLOB types have length limits, but the strings are
not padded by the system. Note that if you use a large length for a
VARCHAR or CLOB type, no extra space is used in the database. The space
used for each stored item is proportional to its actual length.</p>
<p>If CHARACTER is used without specifying the length, the length
defaults to 1. For the CLOB type, the length limit can be defined in
units of kilobyte (K, 1024), megabyte (M, 1024 * 1024) or gigabyte (G,
1024 * 1024 * 1024), using the <code class="literal"><multiplier></code>. If
CLOB is used without specifying the length, the length defaults to
1GB.</p>
<p>
<code class="literal"><character string type> ::= { CHARACTER | CHAR }
[ <left paren> <character length> <right paren> ] | {
CHARACTER VARYING | CHAR VARYING | VARCHAR } <left paren>
<character length> <right paren> | LONGVARCHAR [ <left
paren> <character length> <right paren> ] | <character
large object type></code>
</p>
<p>
<code class="literal"><character large object type> ::= { CHARACTER
LARGE OBJECT | CHAR LARGE OBJECT | CLOB } [ <left paren>
<character large object length> <right paren>
]</code>
</p>
<p>
<code class="literal"><character length> ::= <unsigned integer>
[ <char length units> ]</code>
</p>
<p>
<code class="literal"><large object length> ::= <length> [
<multiplier> ] | <large object length
token></code>
</p>
<p>
<code class="literal"><character large object length> ::= <large
object length> [ <char length units> ]</code>
</p>
<p>
<code class="literal"><large object length token> ::= <digit>...
<multiplier></code>
</p>
<p>
<code class="literal"><multiplier> ::= K | M | G </code>
</p>
<p>
<code class="literal"><char length units> ::= CHARACTERS |
OCTETS</code>
</p>
<p>Each character type has a collation. This is either a default
collation or stated explicitly with the COLLATE clause. Collations are
discussed in the <a class="link" href="databaseobjects-chapt.html" title="Chapter 4. Schemas and Database Objects">Schemas and Database Objects</a> chapter.</p>
<pre class="programlisting"> CHAR(10)
CHARACTER(10)
VARCHAR(2)
CHAR VARYING(2)
CLOB(1000)
CLOB(30K)
CHARACTER LARGE OBJECT(1M)
LONGVARCHAR
</pre>
</div>
<div class="section" title="Binary String Types">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_binary_types"></a>Binary String Types</h3>
</div>
</div>
</div>
<a name="N1054B" class="indexterm"></a>
<p>The BINARY, BINARY VARYING and BLOB types are the SQL Standard
binary string types. VARBINARY and BINARY LARGE OBJECT are synonyms for
BINARY VARYING and BLOB types. HyperSQL also supports LONGVARBINARY as a
synonym for VARBINARY. You can set LONGVARBINARY to map to BLOB, with
the <code class="literal">sql.longvar_is_lob</code> connection property or the SET
DATABASE SQL LONGVAR IS LOB TRUE statement.</p>
<p>Binary string types are used in a similar way to character string
types. There are several built-in functions that are overloaded to
support character, binary and bit strings.</p>
<p>The BINARY type represents a fixed width-string. Each shorter
string is padded with zeros to fill the fixed width. Similar to the
CHARACTER type, the trailing zeros in the BINARY string are simply
discarded in some operations. For the same reason, it is best to avoid
this particular type and use VARBINARY instead.</p>
<p>When two binary values are compared, if one is of BINARY type,
then zero padding is performed to extend the length of the shorter
string to the longer one before comparison. No padding is performed with
other binary types. If the bytes compare equal to the end of the shorter
value, then the longer string is considered larger than the shorter
string.</p>
<p>If BINARY is used without specifying the length, the length
defaults to 1. For the BLOB type, the length limit can be defined in
units of kilobyte (K, 1024), megabyte (M, 1024 * 1024) or gigabyte (G,
1024 * 1024 * 1024), using the <code class="literal"><multiplier></code>. If
BLOB is used without specifying the length, the length defaults to
1GB.</p>
<p>The UUID type represents a UUID string. The type is similar to
BINARY(16) but with the extra enforcement that disallows assigning,
casting or compareing with shorter or longer strings. Strings such as
'24ff1824-01e8-4dac-8eb3-3fee32ad2b9c' or
'24ff182401e84dac8eb33fee32ad2b9c' are allowed. When a value of the UUID
type is converted to a CHARACTER type, the hyphens are inserted in the
required positions.</p>
<p>
<code class="literal"><binary string type> ::= BINARY [ <left
paren> <length> <right paren> ] | { BINARY VARYING |
VARBINARY } <left paren> <length> <right paren> |
LONGVARBINARY [ <left paren> <length> <right paren> ]
| UUID | <binary large object string type></code>
</p>
<p>
<code class="literal"><binary large object string type> ::= { BINARY
LARGE OBJECT | BLOB } [ <left paren> <large object length>
<right paren> ]</code>
</p>
<p>
<code class="literal"><length> ::= <unsigned
integer></code>
</p>
<pre class="programlisting"> BINARY(10)
VARBINARY(2)
BINARY VARYING(2)
BLOB(1000)
BLOB(30G)
BINARY LARGE OBJECT(1M)
LONGVARBINARY
</pre>
</div>
<div class="section" title="Bit String Types">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_bit_types"></a>Bit String Types</h3>
</div>
</div>
</div>
<a name="N10571" class="indexterm"></a>
<p>The BIT and BIT VARYING types are the supported bit string types.
These types were defined by SQL:1999 but were later removed from the
Standard. Bit types represent bit maps of given lengths. Each bit is 0
or 1. The BIT type represents a fixed width-string. Each shorter string
is padded with zeros to fill the fixed with. If BIT is used without
specifying the length, the length defaults to 1. The BIT VARYING type
has a maximum width and shorter strings are not padded.</p>
<p>Before the introduction of the BOOLEAN type to the SQL Standard, a
single-bit string of the type BIT(1) was commonly used. For
compatibility with other products that do not conform to, or extend, the
SQL Standard, HyperSQL allows values of BIT and BIT VARYING types with
length 1 to be converted to and from the BOOLEAN type. BOOLEAN TRUE is
considered equal to B'1', BOOLEAN FALSE is considered equal to
B'0'.</p>
<p>For the same reason, numeric values can be assigned to columns and
variables of the type BIT(1). For assignment, the numeric value zero is
converted to B'0', while all other values are converted to B'1'. For
comparison, numeric values 1 is considered equal to B'1' and numeric
value zero is considered equal to B'0'.</p>
<p>It is not allowed to perform other arithmetic or boolean
operations involving BIT(1) and BIT VARYING(1). The kid of operations
allowed on bit strings are analogous to those allowed on BINARY and
CHARACTER strings. Several built-in functions support all three types of
string.</p>
<p>
<code class="literal"><bit string type> ::= BIT [ <left paren>
<length> <right paren> ] | BIT VARYING <left paren>
<length> <right paren></code>
</p>
<pre class="programlisting"> BIT
BIT(10)
BIT VARYING(2)
</pre>
</div>
<div class="section" title="Lob Data">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_lob_data"></a>Lob Data</h3>
</div>
</div>
</div>
<p>BLOB and CLOB are lob types. These types are used for very long
strings that do not necessarily fit in memory. Small lobs that fit in
memory can be accessed just like BINARY or VARCHAR column data. But lobs
are usually much larger and therefore accessed with special JDBC
methods.</p>
<p>To insert a lob into a table, or to update a column of lob type
with a new lob, you can use the <code class="literal">setBinaryStream()</code> and
<code class="literal">setCharacterStream()</code> methods of JDBC
<code class="literal">java.sql.PreparedStatement</code>. These are very efficient
methods for long lobs. Other methods are also supported. If the data for
the BLOB or CLOB is already a memory object, you can use the
<code class="literal">setBytes()</code> or <code class="literal">setString()</code> methods,
which are efficient for memory data. Another method is to obtain a lob
with the <code class="literal">getBlob()</code> and <code class="literal">getClob()</code>
methods of <code class="literal">java.sql.Connection</code>, populate its data,
then use the <code class="literal">setBlob()</code> or
<code class="literal">setClob()</code> methods of
<code class="literal">PreparedStatement</code>. Yet another method allows to
create instances of <code class="literal">org.hsqldb.jdbc.JDBCBlobFile</code> and
<code class="literal">org.hsqldb.jdbc.JDBCClobFile</code> and construct a large
lob for use with <code class="literal">setBlob()</code> and
<code class="literal">setClob()</code> methods.</p>
<p>A lob is retrieved from a ResultSet with the
<code class="literal">getBlob()</code> or <code class="literal">getClob()</code> method. The
steaming methods of the lob objects are then used to access the data.
HyperSQL also allows efficient access to chunks of lobs with
<code class="literal">getBytes()</code> or <code class="literal">getString()</code> methods.
Furthermore, parts of a BLOB or CLOB already stored in a table can be
modified. An updatable <code class="literal">ResultSet</code> is used to select
the row from the table. The <code class="literal">getBlob()</code> or
<code class="literal">getClob()</code> methods of <code class="literal">ResultSet</code> are
used to access the lob as a <code class="literal">java.sql.Blob</code> or
<code class="literal">java.sql.Clob</code> object. The
<code class="literal">setBytes()</code> and <code class="literal">setString()</code> methods
of these objects can be used to modify the lob. Finally the
<code class="literal">updateRow()</code> method of the
<code class="literal">ResultSet</code> is used to update the lob in the row. Note
these modifications are not allowed with compressed or encrypted
lobs.</p>
<p>Lobs are logically stored in columns of tables. Their physical
storage is a separate *.lobs file. This file is created as soon as a
BLOB or CLOB is inserted into the database. The file will grow as new
lobs are inserted into the database. In version 2, the *.lobs file is
never deleted even if all lobs are deleted from the database. In this
case you can delete the *.lobs file after a SHUTDOWN. When a CHECKPOINT
happens, the space used for deleted lobs is freed and is reused for
future lobs. By default, clobs are stored without compression. You can
use a database setting to enable compression of clobs. This can
significantly reduce the storage size of clobs.</p>
</div>
<div class="section" title="Storage and Handling of Java Objects">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_java_objects"></a>Storage and Handling of Java Objects</h3>
</div>
</div>
</div>
<a name="N105EA" class="indexterm"></a>
<p>From version 2.3.4 there are two options for storing Java
Objects.</p>
<p>The default option allows storing Serializable object. The objects
remain serialized inside the database until they are retrieved. The
application program that retrieves the object must include in its
classpath the Java Class for the object, otherwise it cannot retrieve
the object.</p>
<p>Any serializable Java Object can be inserted directly into a
column of type OTHER using any variation of
<code class="methodname">PreparedStatement.setObject()</code> methods.</p>
<p>The alternative Live Object option is for <code class="literal">mem:</code>
databases only and is enabled when the database property
sql.live_object=true is appended to the connection property that creates
the mem database. For example
<code class="literal">'jdbc:hsqldb:mem:mydb;sql.live_object=true'</code>. With
this option, any Java object can be stored as it is not serialized. The
SQL statement <code class="literal">SET DATABASE SQL LIVE OBJECT TRUE</code> can
be also used. Note the SQL statement must be executed on the first
connection to the database before any data is inserted. No data access
should be made from this connection. Instead, new connections should be
used for data access.</p>
<p>For comparison purposes and in indexes, any two Java Objects are
considered equal unless one of them is NULL. You cannot search for a
specific object or perform a join on a column of type OTHER.</p>
<p>Java Objects can simply be stored internally and no operations can
be performed on them other than assignment between columns of type OTHER
or checking for NULL. Tests such as <code class="literal">WHERE object1 = object2
</code>do not mean what you might expect, as any non-null object
would satisfy such a tests. But <code class="literal">WHERE object1 IS NOT
NULL</code> is perfectly acceptable.</p>
<p>The engine does not allow normal column values to be assigned to
Java Object columns (for example, assigning an INTEGER or STRING to such
a column with an SQL statement such as <code class="literal">UPDATE mytable SET
objectcol = intcol WHERE ...</code>).</p>
<p>
<code class="literal"><java object type> ::= OTHER</code>
</p>
<p>The default method of storage is used when the objects and their
state needs to be saved and retrieved in the future. This method is also
used when memory resources are limited and collections of objects are
stored and retrieved only when needed.</p>
<p>The Live Object option uses the database table as a collection of
objects. This allows storing some attributes of the objects in the same
table alongside the object itself and fast search and retrieval of
objects on their attributes. For example when many thousands of live
objects contain details of films, The film title and the director can be
stored in the table and searches can be performed for films on these
attributes:</p>
<div class="informalexample">
<pre class="programlisting">CREATE TABLE movies (director VARCHAR(30), title VARCHAR(40), obj OTHER)
SELECT obj FROM movies WHERE director LIKE 'Luc%'
</pre>
<p>In any case, at least one attribute of the object should be
stored to allow efficient retrieval of the objects from both Live
Object and Serialized storage. Often an id number is used a the
attribute.</p>
</div>
</div>
<div class="section" title="Type Length, Precision and Scale">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_length_precision"></a>Type Length, Precision and Scale</h3>
</div>
</div>
</div>
<p>In older version of HyperSQL, all table column type definitions
with a column length, precision or scale qualifier were accepted and
ignored. HSQLDB 1.8 enforced correctness but included an option to
enforce the length, precision or scale.</p>
<p>In HyperSQL 2, length, precision and scale qualifiers are always
enforced. For backward compatibility, when older databases which had the
property hsqldb.enforce_strict_size=false are converted to version 2,
this property is retained. However, this is a temporary measure. You
should test your application to ensure the length, precision and scale
that is used for column definitions is appropriate for the application
data. You can test with the default database setting, which enforces the
sizes.</p>
<p>String types, including all BIT, BINARY and CHAR string types plus
CLOB and BLOB, are generally defined with a length. If no length is
specified for BIT, BINARY and CHAR, the default length is 1. For CLOB
and BLOB an implementation defined length of 1M is used.</p>
<p>TIME and TIMESTAMP types can be defined with a fractional second
precision between 0 and 9. INTERVAL type definition may have precision
and, in some cases, fraction second precision. DECIMAL and NUMERIC types
may be defined with precision and scale. For all of these types a
default precision or scale value is used if one is not specified. The
default scale is 0. The default fractional precision for TIME is 0,
while it is 6 for TIMESTAMP.</p>
<p>Values can be converted from one type to another in two different
ways: by using explicit CAST expression or by implicit conversion used
in assignment, comparison and aggregation.</p>
<p>String values cannot be assigned to VARCHAR columns if they are
longer than the defined type length. For CHARACTER columns, a long
string can be assigned (with truncation) only if all the characters
after the length are spaces. Shorter strings are padded with the space
character when inserted into a CHARACTER column. Similar rules are
applied to VARBINARY and BINARY columns. For BINARY columns, the padding
and truncation rules are applied with zero bytes, instead of
spaces.</p>
<p>Explicit CAST of a value to a CHARACTER or VARCHAR type will
result in forced truncation or padding. So a test such as <code class="literal">CAST
(mycol AS VARCHAR(2)) = 'xy'</code> will find the values beginning
with 'xy'. This is the equivalent of <code class="literal">SUBSTRING(mycol FROM 1 FOR
2)= 'xy'</code>.</p>
<p>For all numeric types, the rules of explicit cast and implicit
conversion are the same. If cast or conversion causes any digits to be
lost from the fractional part, it can take place. If the non-fractional
part of the value cannot be represented in the new type, cast or
conversion cannot take place and will result in a data exception.</p>
<p>There are special rules for DATE, TIME, TIMESTAMP and INTERVAL
casts and conversions.</p>
</div>
</div>
<div class="section" title="Datetime types">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_datetime_types"></a>Datetime types</h2>
</div>
</div>
</div>
<p>HSQLDB fully supports datetime and interval types and operations,
including all relevant optional features, as specified by the SQL Standard
since SQL-92. The two groups of types are complementary.</p>
<a name="N10641" class="indexterm"></a>
<p>The DATE type represents a calendar date with YEAR, MONTH and DAY
fields.</p>
<p>The TIME type represents time of day with HOUR, MINUTE and SECOND
fields, plus an optional SECOND FRACTION field.</p>
<p>The TIMESTAMP type represents the combination of DATE and TIME
types.</p>
<p>TIME and TIMESTAMP types can include WITH TIME ZONE or WITHOUT TIME
ZONE (the default) qualifiers. They can have fractional second parts. For
example, TIME(6) has six fractional digits for the second field.</p>
<p>If fractional second precision is not specified, it defaults to 0
for TIME and to 6 for TIMESTAMP.</p>
<p>
<code class="literal"><datetime type> ::= DATE | TIME [ <left
paren> <time precision> <right paren> ] [ <with or
without time zone> ] | TIMESTAMP [ <left paren> <timestamp
precision> <right paren> ] [ <with or without time zone>
]</code>
</p>
<p>
<code class="literal"><with or without time zone> ::= WITH TIME ZONE |
WITHOUT TIME ZONE</code>
</p>
<p>
<code class="literal"><time precision> ::= <time fractional seconds
precision></code>
</p>
<p>
<code class="literal"><timestamp precision> ::= <time fractional
seconds precision></code>
</p>
<p>
<code class="literal"><time fractional seconds precision> ::=
<unsigned integer></code>
</p>
<pre class="programlisting"> DATE
TIME(6)
TIMESTAMP(2) WITH TIME ZONE
</pre>
<p>TIME or TIMESTAMP literals containing a zone displacement value are
WITH TIME ZONE. Examples of the string literals used to represent date
time values, some with time zone, some without, are below:</p>
<pre class="programlisting"> DATE '2008-08-22'
TIMESTAMP '2008-08-08 20:08:08'
TIMESTAMP '2008-08-08 20:08:08+8:00' /* Beijing */
TIME '20:08:08.034900'
TIME '20:08:08.034900-8:00' /* US Pacific */</pre>
<a name="N10665" class="indexterm"></a>
<p>
<span class="bold"><strong>Time Zone</strong></span>
</p>
<p>DATE values do not take time zones. For example United Nations
designates 5 June as World Environment Day, which was observed on DATE
'2008-06-05' in different time zones.</p>
<p>TIME and TIMESTAMP values without time zone, usually have a context
that indicates some local time zone. For example, a database for college
course timetables usually stores class dates and times without time zones.
This works because the location of the college is fixed and the time zone
displacement is the same for all the values. Even when the events take
place in different time zones, for example international flight times, it
is possible to store all the datetime information as references to a
single time zone, usually GMT. For some databases it may be useful to
store the time zone displacement together with each datetime value. SQL’s
TIME WITH TIME ZONE and TIMESTAMP WITH TIME ZONE values include a time
zone displacement value.</p>
<p>The time zone displacement is of the type INTERVAL HOUR TO MINUTE.
This data type is described in the next section. The legal values are
between '–18:00' and '+18:00'.</p>
<a name="N10674" class="indexterm"></a>
<p>
<span class="bold"><strong>Operations on Datetime
Types</strong></span>
</p>
<p>The expression <code class="literal"><datetime expression> AT TIME ZONE
<time displacement></code> evaluates to a datetime value
representing exactly the same point of time in the specified
<code class="literal"><time displacement></code>. The expression, <code class="literal">AT
LOCAL</code> is equivalent to <code class="literal">AT TIME ZONE <local time
displacement></code>. If <code class="literal">AT TIME ZONE</code> is used
with a datetime operand of type WITHOUT TIME ZONE, the operand is first
converted to a value of type WITH TIME ZONE at the session’s time
displacement, then the specified time zone displacement is set for the
value. Therefore, in these cases, the final value depends on the time zone
of the session in which the statement was used.</p>
<p>AT TIME ZONE, modifies the field values of the datetime operand.
This is done by the following procedure:</p>
<div class="orderedlist">
<ol class="orderedlist" type="1">
<li class="listitem">
<p>determine the corresponding datetime at UTC.</p>
</li>
<li class="listitem">
<p>find the datetime value at the given time zone that corresponds
with the UTC value from step 1.</p>
</li>
</ol>
</div>
<p>Example a:</p>
<pre class="programlisting"> TIME '12:00:00' AT TIME ZONE INTERVAL '1:00' HOUR TO MINUTE
</pre>
<p>If the session’s time zone displacement is -'8:00', then in step 1,
TIME '12:00:00' is converted to UTC, which is TIME '20:00:00+0:00'. In
step 2, this value is expressed as TIME '21:00:00+1:00'.</p>
<p>Example b:</p>
<pre class="programlisting"> TIME '12:00:00-5:00' AT TIME ZONE INTERVAL '1:00' HOUR TO MINUTE
</pre>
<p>Because the operand has a time zone, the result is independent of
the session time zone displacement. Step 1 results in TIME
'17:00:00+0:00', and step 2 results in TIME '18:00:00+1:00'</p>
<p>Note that the operand is not limited to datetime literals used in
these examples. Any valid expression that evaluates to a datetime value
can be the operand.</p>
<p>
<span class="bold"><strong>Type Conversion</strong></span>
</p>
<p>CAST is used for all other conversions. Examples:</p>
<pre class="programlisting"> CAST (<value> AS TIME WITHOUT TIME ZONE)
CAST (<value> AS TIME WITH TIME ZONE)</pre>
<p>In the first example, if <code class="literal"><value></code> has a time
zone component, it is simply dropped. For example TIME '12:00:00-5:00' is
converted to TIME '12:00:00'</p>
<p>In the second example, if <code class="literal"><value></code> has no
time zone component, the current time zone displacement of the session is
added. For example TIME '12:00:00' is converted to TIME '12:00:00-8:00'
when the session time zone displacement is '-8:00'.</p>
<p>Conversion between DATE and TIMESTAMP is performed by removing the
TIME component of a TIMESTAMP value or by setting the hour, minute and
second fields to zero. TIMESTAMP '2008-08-08 20:08:08+8:00' becomes DATE
'2008-08-08', while DATE '2008-08-22' becomes TIMESTAMP '2008-08-22
00:00:00'.</p>
<p>Conversion between TIME and TIMESTAMP is performed by removing the
DATE field values of a TIMESTAMP value or by appending the fields of the
TIME value to the fields of the current session date value.</p>
<p>
<span class="bold"><strong>Assignment</strong></span>
</p>
<p>When a value is assigned to a datetime target, e.g., a value is used
to update a row of a table, the type of the value must be the same as the
target, but the WITH TIME ZONE or WITHOUT TIME ZONE characteristics can be
different. If the types are not the same, an explicit CAST must be used to
convert the value into the target type.</p>
<p>
<span class="bold"><strong>Comparison</strong></span>
</p>
<p>When values WITH TIME ZONE are compared, they are converted to UTC
values before comparison. If a value WITH TIME ZONE is compared to another
WITHOUT TIME ZONE, then the WITH TIME ZONE value is converted to AT LOCAL,
then converted to WITHOUT TIME ZONE before comparison.</p>
<p>It is not recommended to design applications that rely on
comparisons and conversions between TIME values WITH TIME ZONE. The
conversions may involve normalisation of the time value, resulting in
unexpected results. For example, the expression: BETWEEN(TIME
'12:00:00-8:00', TIME '22:00:00-8:00') is converted to BETWEEN(TIME
'20:00:00+0:00', TIME '06:00:00+0:00') when it is evaluated in the UTC
zone, which is always FALSE.</p>
<p>
<span class="bold"><strong>Functions</strong></span>
</p>
<p>Several functions return the current session timestamp in different
datetime types:</p>
<div class="informaltable">
<table cellspacing="0" style="border-collapse: collapse;border-top: 0.5pt solid ; border-bottom: 0.5pt solid ; border-left: 0.5pt solid ; border-right: 0.5pt solid ; ">
<colgroup>
<col>
<col>
</colgroup>
<tbody>
<tr>
<td style="border-right: 0.5pt solid ; border-bottom: 0.5pt solid ; ">
<p>CURRENT_DATE</p>
</td><td style="border-bottom: 0.5pt solid ; ">
<p>DATE</p>
</td>
</tr>
<tr>
<td style="border-right: 0.5pt solid ; border-bottom: 0.5pt solid ; ">
<p>CURRENT_TIME</p>
</td><td style="border-bottom: 0.5pt solid ; ">
<p>TIME WITH TIME ZONE</p>
</td>
</tr>
<tr>
<td style="border-right: 0.5pt solid ; border-bottom: 0.5pt solid ; ">
<p>CURRENT_TIMESTAMP</p>
</td><td style="border-bottom: 0.5pt solid ; ">
<p>TIMESTAMP WITH TIME ZONE</p>
</td>
</tr>
<tr>
<td style="border-right: 0.5pt solid ; border-bottom: 0.5pt solid ; ">
<p>LOCALTIME</p>
</td><td style="border-bottom: 0.5pt solid ; ">
<p>TIME WITHOUT TIME ZONE</p>
</td>
</tr>
<tr>
<td style="border-right: 0.5pt solid ; ">
<p>LOCALTIMESTAMP</p>
</td><td style="">
<p>TIMESTAMP WITHOUT TIME ZONE</p>
</td>
</tr>
</tbody>
</table>
</div>
<p>HyperSQL supports a very extensive range of functions for
conversion, extraction and manipulation of DATE and TIMESTAMP values. See
the <a class="link" href="builtinfunctions-chapt.html" title="Chapter 10. Built In Functions">Built In Functions</a> chapter.</p>
<p>
<span class="bold"><strong>Session Time Zone
Displacement</strong></span>
</p>
<p>When an SQL session is started (with a JDBC connection) the local
time zone of the client JVM (including any seasonal time adjustments such
as daylight saving time) is used as the session time zone displacement.
Note that the SQL session time displacement is not changed when a seasonal
time adjustment takes place while the session is open. To change the SQL
session time zone displacement use the following commands:</p>
<p>
<code class="literal">SET TIME ZONE <time
displacement></code>
</p>
<p>
<code class="literal">SET TIME ZONE LOCAL</code>
</p>
<p>The first command sets the displacement to the given value. The
second command restores the original, real time zone displacement of the
session.</p>
<p>
<span class="bold"><strong>Datetime Values and
Java</strong></span>
</p>
<p>When datetime values are sent to the database using the
<code class="classname">PreparedStatement</code> or
<code class="classname">CallableStatement</code> interfaces, the Java object is
converted to the type of the prepared or callable statement parameter.
This type may be DATE, TIME, or TIMESTAMP (with or without time zone). The
time zone displacement is the time zone of the JDBC session.</p>
<p>When datetime values are retrieved from the database using the
<code class="literal">ResultSet</code> interface, there are two representations. The
<code class="methodname">getString(…)</code> methods of the
<code class="classname">ResultSet</code> interface, return an exact representation
of the value in the SQL type as it is stored in the database. This
includes the correct number of digits for the fractional second field, and
for values with time zone displacement, the time zone displacement.
Therefore if TIME '12:00:00' is stored in the database, all users in
different time zones will get '12:00:00' when they retrieve the value as a
string. The <code class="methodname">getTime(…)</code> and
<code class="methodname">getTimestamp(…)</code> methods of the
<code class="classname">ResultSet</code> interface return Java objects that are
corrected for the session time zone. The UTC millisecond value contained
the <code class="classname">java.sql.Time</code> or
<code class="classname">java.sql.Timestamp</code> objects will be adjusted to the
time zone of the session, therefore the
<code class="methodname">toString()</code> method of these objects return the
same values in different time zones.</p>
<p>If you want to store and retrieve UTC values that are independent of
any session's time zone, you can use a TIMESTAMP WITH TIME ZONE column.
The <code class="methodname">setTime(...)</code> and
<code class="methodname">setTimestamp(...)</code> methods of the
PreparedStatement interface which have a Calendar parameter can be used to
assign the values. The time zone of the given Calendar argument is used as
the time zone. Conversely, the <code class="methodname">getTime(...)</code> and
<code class="methodname">getTimestamp(...)</code> methods of the ResultSet
interface which have a Calendar parameter can be used with a Calendar
argument to retrieve the values.</p>
<p>JDBC 4 and JAVA6 has an unfortunate limitation and does not include
type codes for SQL datetime types that have a TIME ZONE property.
Therefore, for compatibility with database tools that are limited to the
JDBC type codes, HyperSQL reports these types by default as datetime types
without TIME ZONE.</p>
<p>
<span class="bold"><strong>Java 8 Extensions</strong></span>
</p>
<p>JAVA 8 introduced new type codes for TIMESTAMP WITH TIME ZONE and
TIME WITH TIME ZONE. HSQLDB 2.4.0 and later when compiled with JDK8
supports this in <code class="classname">ResultSet</code>,
<code class="classname">PreparedStatement</code> and
<code class="classname">CallableStatement</code>.</p>
<div class="itemizedlist">
<ul class="itemizedlist" type="disc">
<li class="listitem">
<p>The <code class="methodname">getObject(int columnIndex)</code> method
on a column of TIMESTAMP WITH TIME ZONE returns an
<code class="classname">java.time.OffsetDateTime</code> object. </p>
</li>
<li class="listitem">
<p>The <code class="methodname">getObject(int columnIndex)</code> method
on a column of TIME WITH TIME ZONE returns an
<code class="classname">java.time.OffsetTime</code> object.</p>
</li>
<li class="listitem">
<p>The <code class="methodname">getObject(int columnIndex, Class
type)</code> method on any date, time and timestamp supports the
<code class="classname">java.time</code> package types:
<code class="classname">LocalDate</code>, <code class="classname">LocalTime</code>,
<code class="classname">LocalDateTime</code>,
<code class="classname">OffsetTime</code> and
<code class="classname">OffsetDateTime</code> as well as
<code class="classname">java.sql</code> package types,
<code class="classname">Date</code>, <code class="classname">Time</code> and
<code class="classname">Timestamp</code>.</p>
</li>
<li class="listitem">
<p>The <code class="methodname">setObject</code> methods also support Java
objects of the types listed above.</p>
</li>
<li class="listitem">
<p>The <code class="methodname">getObject</code> and
<code class="methodname">setObject</code> methods with column name parameters
bahave just like their counterparts with columnIndexe
parameters.</p>
</li>
</ul>
</div>
<p>
<span class="bold"><strong>Non-Standard
Extensions</strong></span>
</p>
<p>HyperSQL version 2.3 supports some extensions to the SQL standard
treatment of datetime and interval types. For example, the Standard
expression to add a number of days to a date has an explicit INTERVAL
value but HSQLDB also allows an integer to be used without specifying DAY.
Examples of some Standard expressions and their non-standard alternatives
are given below:</p>
<div class="informalexample">
<pre class="programlisting"> -- standard forms
CURRENT_DATE + '2' DAY
SELECT (LOCALTIMESTAMP - atimestampcolumn) DAY TO SECOND FROM atable
-- non-standard forms
CURRENT_DATE + 2
SELECT LOCALTIMESTAMP - atimestampcolumn FROM atable
</pre>
</div>
<p>It is recommended to use the SQL Standard syntax as it is more
precise and avoids ambiguity.</p>
</div>
<div class="section" title="Interval Types">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_interval_typs"></a>Interval Types</h2>
</div>
</div>
</div>
<a name="N107A9" class="indexterm"></a>
<p>Interval types are used to represent differences between date time
values. The difference between two date time values can be measured in
seconds or in months. For measurements in months, the units YEAR and MONTH
are available, while for measurements in seconds, the units DAY, HOUR,
MINUTE, SECOND are available. The units can be used individually, or as a
range. An interval type can specify the precision of the most significant
field and the second fraction digits of the SECOND field (if it has a
SECOND field). The default precision is 2. The default second precision is
0.</p>
<p>
<code class="literal"><interval type> ::= INTERVAL <interval
qualifier></code>
</p>
<p>
<code class="literal"><interval qualifier> ::= <start field> TO
<end field> | <single datetime field></code>
</p>
<p>
<code class="literal"><start field> ::= <non-second primary datetime
field> [ <left paren> <interval leading field precision>
<right paren> ]</code>
</p>
<p>
<code class="literal"><end field> ::= <non-second primary datetime
field> | SECOND [ <left paren> <interval fractional seconds
precision> <right paren> ]</code>
</p>
<p>
<code class="literal"><single datetime field> ::= <non-second primary
datetime field> [ <left paren> <interval leading field
precision> <right paren> ] | SECOND [ <left paren>
<interval leading field precision> [ <comma> <interval
fractional seconds precision> ] <right paren>
]</code>
</p>
<p>
<code class="literal"><primary datetime field> ::= <non-second
primary datetime field> | SECOND</code>
</p>
<p>
<code class="literal"><non-second primary datetime field> ::= YEAR |
MONTH | DAY | HOUR | MINUTE</code>
</p>
<p>
<code class="literal"><interval fractional seconds precision> ::=
<unsigned integer></code>
</p>
<p>
<code class="literal"><interval leading field precision> ::=
<unsigned integer></code>
</p>
<p>Examples of INTERVAL type definition:</p>
<pre class="programlisting"> INTERVAL YEAR TO MONTH
INTERVAL YEAR(3)
INTERVAL DAY(4) TO HOUR
INTERVAL MINUTE(4) TO SECOND(6)
INTERVAL SECOND(4,6)
</pre>
<p>The word INTERVAL indicates the general type name. The rest of the
definition is called an <code class="literal"><interval qualifier></code>.
This designation is important, as in most expressions
<code class="literal"><interval qualifier></code> is used without the word
INTERVAL.</p>
<p>
<span class="bold"><strong>Interval Values</strong></span>
</p>
<p>An interval value can be negative, positive or zero. An interval
type has all the datetime fields in the specified range. These fields are
similar to those in the TIMESTAMP type. The differences are as
follows:</p>
<p>The first field of an interval value can hold any numeric value up
to the specified precision. For example, the hour field in HOUR(2) TO
SECOND can hold values above 23 (up to 99). The year and month fields can
hold zero (unlike a TIMESTAMP value) and the maximum value of a month
field that is not the most significant field, is 11.</p>
<p>The standard function <code class="literal">ABS(<interval value
expression>)</code> can be used to convert a negative interval value
to a positive one.</p>
<p>The literal representation of interval values consists of the type
definition, with a string representing the interval value inserted after
the word INTERVAL. Some examples of interval literal below:</p>
<pre class="programlisting"> INTERVAL '145 23:12:19.345' DAY(3) TO SECOND(3)
INTERVAL '3503:12:19.345' HOUR TO SECOND(3) /* equal to the first value */
INTERVAL '19.345' SECOND(4,3) /* maximum number of digits for the second value is 4, and each value is expressed with three fraction digits. */
INTERVAL '-23-10' YEAR(2) TO MONTH
</pre>
<p>Interval values of the types that are based on seconds can be cast
into one another. Similarly those that are based on months can be cast
into one another. It is not possible to cast or convert a value based on
seconds to one based on months, or vice versa.</p>
<p>When a cast is performed to a type with a smaller least-significant
field, nothing is lost from the interval value. Otherwise, the values for
the missing least-significant fields are discarded. Examples:</p>
<pre class="programlisting"> CAST ( INTERVAL '145 23:12:19' DAY TO SECOND AS INTERVAL DAY TO HOUR ) = INTERVAL '145 23' DAY TO HOUR
CAST(INTERVAL '145 23' DAY TO HOUR AS INTERVAL DAY TO SECOND) = INTERVAL '145 23:00:00' DAY TO SECOND
</pre>
<p>A numeric value can be cast to an interval type. In this case the
numeric value is first converted to a single-field INTERVAL type with the
same field as the least significant field of the target interval type.
This value is then converted to the target interval type For example CAST(
22 AS INTERVAL YEAR TO MONTH) evaluates to INTERVAL '22' MONTH and then
INTERVAL '1 10' YEAR TO MONTH. Note that SQL Standard only supports casts
to single-field INTERVAL types, while HyperSQL allows casting to
multi-field types as well.</p>
<p>An interval value can be cast to a numeric type. In this case the
interval value is first converted to a single-field INTERVAL type with the
same field as the least significant filed of the interval value. The value
is then converted to the target type. For example CAST (INTERVAL '1-11'
YEAR TO MONTH AS INT) evaluates to INTERVAL '23' MONTH, and then
23.</p>
<p>An interval value can be cast into a character type, which results
in an INTERVAL literal. A character value can be cast into an INTERVAL
type so long as it is a string with a format compatible with an INTERVAL
literal.</p>
<p>Two interval values can be added or subtracted so long as the types
of both are based on the same field, i.e., both are based on MONTH or
SECOND. The values are both converted to a single-field interval type with
same field as the least-significant field between the two types. After
addition or subtraction, the result is converted to an interval type that
contains all the fields of the two original types.</p>
<p>An interval value can be multiplied or divided by a numeric value.
Again, the value is converted to a numeric, which is then multiplied or
divided, before converting back to the original interval type.</p>
<p>An interval value is negated by simply prefixing with the minus
sign.</p>
<p>Interval values used in expressions are either typed values,
including interval literals, or are interval casts. The expression:
<code class="literal"><expression> <interval qualifier></code> is a cast
of the result of the <code class="literal"><expression></code> into the
INTERVAL type specified by the <code class="literal"><interval qualifier>. The
cast can be formed by adding the keywords and parentheses as follows: CAST
( <expression> AS INTERVAL <interval qualifier>
).</code>
</p>
<p>
<code class="literal">The examples below feature different forms of expression
that represent an interval value, which is then added to the given date
literal.</code>
</p>
<pre class="programlisting"> DATE '2000-01-01' + INTERVAL '1-10' YEAR TO MONTH /* interval literal */
DATE '2000-01-01' + '1-10' YEAR TO MONTH /* the string '1-10' is cast into INTERVAL YEAR TO MONTH */
DATE '2000-01-01' + 22 MONTH /* the integer 22 is cast into INTERVAL MONTH, same value as above */
DATE '2000-01-01' - 22 DAY /* the integer 22 is cast into INTERVAL DAY */
DATE '2000-01-01' + COL2 /* the type of COL2 must be an INTERVAL type */
DATE '2000-01-01' + COL2 MONTH /* COL2 may be a number, it is cast into a MONTH interval */
</pre>
<p>
<span class="bold"><strong>Datetime and Interval
Operations</strong></span>
</p>
<p>An interval can be added to or subtracted from a datetime value so
long as they have some fields in common. For example, an INTERVAL MONTH
cannot be added to a TIME value, while an INTERVAL HOUR TO SECOND can. The
interval is first converted to a numeric value, then the value is added
to, or subtracted from, the corresponding field of the datetime
value.</p>
<p>If the result of addition or subtraction is beyond the permissible
range for the field, the field value is normalised and carried over to the
next significant field until all the fields are normalised. For example,
adding 20 minutes to TIME '23:50:10' will result successively in
'23:70:10', '24:10:10' and finally TIME '00:10:10'. Subtracting 20 minutes
from the result is performed as follows: '00:-10:10', '-1:50:10', finally
TIME '23:50:10'. Note that if DATE or TIMESTAMP normalisation results in
the YEAR field value out of the range (1,1000), then an exception
condition is raised.</p>
<p>If an interval value based on MONTH is added to, or subtracted from
a DATE or TIMESTAMP value, the result may have an invalid day (30 or 31)
for the given result month. In this case an exception condition is
raised.</p>
<p>The result of subtraction of two datetime expressions is an interval
value. The two datetime expressions must be of the same type. The type of
the interval value must be specified in the expression, using only the
interval field names. The two datetime expressions are enclosed in
parentheses, followed by the <code class="literal"><interval qualifier></code>
fields. In the first example below, COL1 and COL2 are of the same datetime
type, and the result is evaluated in INTERVAL YEAR TO MONTH type.</p>
<pre class="programlisting"> (COL1 – COL2) YEAR TO MONTH /* the difference between two DATE or two TIEMSTAMP values in years and months */
(CURRENT_DATE – COL3) DAY /* the number of days between the value of COL3 and the current date */
(CURRENT_DATE - DATE '2000-01-01') YEAR TO MONTH /* the number of years and months since the beginning of this century */
CURRENT_DATE - 2 DAY /* the date of the day before yesterday */
(CURRENT_TIMESTAMP - TIMESTAMP '2009-01-01 00:00:00') DAY(4) TO SECOND(2) /* days to seconds since the given date */
</pre>
<p>The individual fields of both datetime and interval values can be
extracted using the EXTRACT function. The same function can also be used
to extract the time zone displacement fields of a datetime value.</p>
<p>
<code class="literal">EXTRACT ({YEAR | MONTH | DAY | HOUR | MINUTE | SECOND |
TIMEZONE_HOUR | TIMEZONE_MINUTE | DAY_OF_WEEK | WEEK_OF_YEAR } FROM
{<datetime value> | <interval value>})</code>
</p>
<p>The dichotomy between interval types based on seconds, and those
based on months, stems from the fact that the different calendar months
have different numbers of days. For example, the expression, “nine months
and nine days since an event” is not exact when the date of the event is
unknown. It can represent a period of around 284 days give or take one.
SQL interval values are independent of any start or end dates or times.
However, when they are added to or subtracted from certain date or
timestamp values, the result may be invalid and cause an exception (e.g.
adding one month to January 30 results in February 30, which is
invalid).</p>
<p>JDBC has an unfortunate limitation and does not include type codes
for SQL INTERVAL types. Therefore, for compatibility with database tools
that are limited to the JDBC type codes, HyperSQL reports these types by
default as VARCHAR. You can use the URL property
<code class="literal">hsqldb.translate_dti_types=false</code> to override the
default behaviour.</p>
<p>
<span class="bold"><strong>Java 8 Extensions</strong></span>
</p>
<p>JAVA 8 does not have a type codes for INTERVAL types. HSQLDB 2.4.0
and later when compiled with JDK8 supports
<code class="classname">java.time</code> types for INTERVAL types in
<code class="classname">ResultSet</code>, <code class="classname">PreparedStatement</code>
and <code class="classname">CallableStatement</code>.</p>
<div class="itemizedlist">
<ul class="itemizedlist" type="disc">
<li class="listitem">
<p>The <code class="methodname">getObject(int columnIndex, Class
type)</code> method on an INTERVAL supports
<code class="classname">java.time.Period</code> type for YEAR and MONTH
interval and <code class="classname">java.time.Duration</code> type for other
interval types that cover DAY to SECOND.</p>
</li>
<li class="listitem">
<p>The <code class="methodname">setObject(int columnIndex)</code> method
accepts <code class="classname">java.time.Period</code> and
<code class="classname">java.time.Duration</code> objects for columns of
relevant INTERVAL types.</p>
</li>
<li class="listitem">
<p>The <code class="methodname">getObject</code> and
<code class="methodname">setObject</code> methods with column name parameters
bahave just like their counterparts with columnIndexe
parameters.</p>
</li>
</ul>
</div>
</div>
<div class="section" title="Arrays">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_array"></a>Arrays</h2>
</div>
</div>
</div>
<p>Array are a powerful feature of SQL:2008 and can help solve many
common problems. Arrays should not be used as a substitute for
tables.</p>
<p>HyperSQL supports arrays of values according to the SQL:2008
Standard.</p>
<p>Elements of the array are either NULL, or of the same data type. It
is possible to define arrays of all supported types, including the types
covered in this chapter and user-defined types, except LOB types. An SQL
array is one dimensional and is addressed from position 1. An empty array
can also be used, which has no element.</p>
<p>Arrays can be stored in the database, as well as being used as
temporary containers of values for simplifying SQL statements. They
facilitate data exchange between the SQL engine and the user's
application.</p>
<p>The full range of supported syntax allows array to be created, used
in SELECT or other statements, combined with rows of tables and used in
routine calls.</p>
<div class="section" title="Array Definition">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_array_def"></a>Array Definition</h3>
</div>
</div>
</div>
<p>The type of a table column, a routine parameter, a variable, or
the return value of a function can be defined as an array.</p>
<p>
<code class="literal"><array type> ::= <data type> ARRAY [ <left
bracket or trigraph> <maximum cardinality> <right bracket or
trigraph> ]</code>
</p>
<p>The word ARRAY is added to any valid type definition except BLOB
and CLOB type definitions. If the optional <code class="literal"><maximum
cardinality></code> is not used, the default value is 1024. The
size of the array cannot be extended beyond maximum cardinality.</p>
<p>In the example below, the table contains a column of integer
arrays and a column of varchar arrays. The VARCHAR array has an explicit
maximum size of 10, which means each array can have between 0 and 10
elements. The INTEGER array has the default maximum size of 1024. The
scores column has a default clause with an empty array. The default
clause can be defined only as <code class="literal">DEFAULT NULL</code> or
<code class="literal">DEFAULT ARRAY[]</code> and does not allow arrays containing
elements.</p>
<div class="informalexample">
<pre class="programlisting"> CREATE TABLE t (id INT PRIMARY KEY, scores INT ARRAY DEFAULT ARRAY[], names VARCHAR(20) ARRAY[10])</pre>
</div>
<p>An array can be constructed from value expressions or a query
expression.</p>
<p>
<code class="literal"><array value constructor by enumeration> ::= ARRAY
<left bracket or trigraph> <array element list> <right
bracket or trigraph></code>
</p>
<p>
<code class="literal"><array element list> ::= <value expression> [
{ <comma> <value expression> }... ]</code>
</p>
<p>
<code class="literal"><array value constructor by query> ::= ARRAY
<left paren> <query expression> [ <order by clause> ]
<right paren></code>
</p>
<p>In the examples below, arrays are constructed from values, column
references or variables, function calls, or query expressions.</p>
<div class="informalexample">
<pre class="programlisting"> ARRAY [ 1, 2, 3 ]
ARRAY [ 'HOT', 'COLD' ]
ARRAY [ var1, var2, CURRENT_DATE ]
ARRAY (SELECT lastname FROM namestable ORDER BY id)
</pre>
</div>
<p>Inserting and updating a table with an ARRAY column can use array
constructors, not only for updated column values, but also in equality
search conditions:</p>
<div class="informalexample">
<pre class="programlisting"> INSERT INTO t VALUES 10, ARRAY[1,2,3], ARRAY['HOT', 'COLD']
UPDATE t SET names = ARRAY['LARGE', 'SMALL'] WHERE id = 12
UPDATE t SET names = ARRAY['LARGE', 'SMALL'] WHERE id < 12 AND scores = ARRAY[3,4]
</pre>
</div>
<p>When using a PreparedStatement with an ARRAY parameter, an object
of the type java.sql.Array must be used to set the parameter. The
<code class="classname">org.hsqldb.jdbc.JDBCArrayBasic</code> class can be used
for constructing a java.sql.Array object in the user's application. Code
fragment below:</p>
<div class="informalexample">
<pre class="programlisting"> String sql = "UPDATE t SET names = ? WHERE id = ?";
PreparedStatement ps = connection.prepareStatement(sql)
Object[] data = new Object[]{"one", "two"};
// default types defined in org.hsqldb.types.Type can be used
org.hsqldb.types.Type type = org.hsqldb.types.Type.SQL_VARCHAR_DEFAULT;
JDBCArrayBasic array = new JDBCArrayBasic(data, type);
ps.setArray(1, array);
ps.setInt(2, 1000);
ps.executeUpdate();
</pre>
</div>
<div class="section" title="Trigraph">
<div class="titlepage">
<div>
<div>
<h4 class="title">
<a name="sgc_trigraph"></a>Trigraph</h4>
</div>
</div>
</div>
<p>A trigraph is a substitute for <left bracket> and
<right bracket>.</p>
<p>
<code class="literal"><left bracket trigraph> ::= ??( </code>
</p>
<p>
<code class="literal"><right bracket trigraph> ::= ??)</code>
</p>
<p>The example below shows the use of trigraphs instead of
brackets.</p>
<div class="informalexample">
<pre class="programlisting"> INSERT INTO t VALUES 10, ARRAY??(1,2,3??), ARRAY['HOT', 'COLD']
UPDATE t SET names = ARRAY ??('LARGE', 'SMALL'??) WHERE id = 12
UPDATE t SET names = ARRAY['LARGE', 'SMALL'] WHERE id < 12 AND scores = ARRAY[3,4]
</pre>
</div>
</div>
</div>
<div class="section" title="Array Reference">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_array_ref"></a>Array Reference</h3>
</div>
</div>
</div>
<p>The most common operations on an array are element reference and
assignment, which are used when reading or writing an element of the
array. Unlike Java and many other languages, arrays are extended if an
element is assigned to an index beyond the current length. This can
result in gaps containing NULL elements. Array length cannot exceed the
maximum cardinality.</p>
<p>Elements of all arrays, including those that are the result of
function calls or other operations can be referenced for reading.</p>
<p>
<code class="literal"><array element reference> ::= <array value
expression> <left bracket> <numeric value expression>
<right bracket></code>
</p>
<p>Elements of arrays that are table columns or routine variables can
be referenced for writing. This is done in a SET statement, either
inside an UPDATE statement, or as a separate statement in the case of
routine variables, OUT and INOUT parameters.</p>
<p>
<code class="literal"><target array element specification> ::= <target
array reference> <left bracket or trigraph> <simple value
specification> <right bracket or trigraph></code>
</p>
<p>
<code class="literal"><target array reference> ::= <SQL parameter
reference> | <column reference></code>
</p>
<p>Note that only simple values or variables are allowed for the
array index when an assignment is performed. The examples below
demonstrates how elements of the array are referenced in SELECT and an
UPDATE statement.</p>
<div class="informalexample">
<pre class="programlisting"> SELECT scores[ranking], names[ranking] FROM t JOIN t1 on (t.id = t1.tid)
UPDATE t SET scores[2] = 123, names[2] = 'Reds' WHERE id = 10
</pre>
</div>
<div class="informalexample">
<pre class="programlisting"> SELECT scores[ranking], names[ranking] FROM t JOIN t1 on (t.id = t1.tid)
UPDATE t SET scores[2] = 123, names[2] = 'Reds' WHERE id = 10
</pre>
</div>
<p></p>
</div>
<div class="section" title="Array Operations">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_array_ops"></a>Array Operations</h3>
</div>
</div>
</div>
<p>Several SQL operations and functions can be used with
arrays.</p>
<p>
<span class="emphasis"><em>CONCATENATION</em></span>
</p>
<p>Array concatenation is performed similar to string concatenation.
All elements of the array on the right are appended to the array on
left.</p>
<p>
<code class="literal"><array concatenation> ::= <array value
expression 1> <concatenation operator> <array value
expression 2></code>
</p>
<p>
<code class="literal"><concatenation operator> ::= ||</code>
</p>
<p>
<span class="emphasis"><em>FUNCTIONS</em></span>
</p>
<p>Seven functions operate on arrays. Details are described in the
<a class="link" href="builtinfunctions-chapt.html" title="Chapter 10. Built In Functions">Built In Functions</a> chapter.</p>
<p>
<code class="literal">ARRAY_AGG</code> is an aggregate function and produces
an array containing values from differnt rows of a SELECT statement.
Details are described in the <a class="link" href="dataaccess-chapt.html" title="Chapter 7. Data Access and Change">Data Access and Change</a> chapter.</p>
<p>
<code class="literal">SEQUENCE_ARRAY</code> creates an array with sequential
elements.</p>
<p>
<code class="literal">CARDINALITY <left paren> <array value
expression> <right paren></code>
</p>
<p>
<code class="literal">MAX_CARDINALITY <left paren> <array value
expression> <right paren></code>
</p>
<p>Array cardinality and max cardinality are functions that return an
integer. CARDINALITY returns the element count, while MAX_CARDINALITY
returns the maximum declared cardinality of an array.</p>
<p>
<code class="literal">POSITION_ARRAY <left paren> <value
expression> IN <array value expression> [FROM <numeric value
expression>] <right paren></code>
</p>
<p>The POSITION_ARRAY function returns the position of the first
match for the <value expression> from the start or from the given
start position when <numeric value expression> is used.</p>
<p>
<code class="literal">TRIM_ARRAY <left paren> <array value
expression> <comma> <numeric value expression> <right
paren></code>
</p>
<p>The TRIM_ARRAY function returns a copy of an array with the
specified number of elements removed from the end of the array. The
<code class="literal"><array value expression></code> can be any expression
that evaluates to an array.</p>
<p>
<code class="literal">SORT_ARRAY <left paren> <array value
expression> [ { ASC | DESC } ] [ NULLS { FIRST | LAST } ] <right
paren></code>
</p>
<p>The SORT_ARRAY function returns a sorted copy of an array. NULL
elements appear at the beginning of the new array. You can change the
sort direction or the position of NULL elements with the option
keywords.</p>
<p>
<span class="emphasis"><em>CAST</em></span>
</p>
<p>An array can be cast into an array of a different type. Each
element of the array is cast into the element type of the target array
type.</p>
<p>
<span class="emphasis"><em>UNNEST</em></span>
</p>
<p>Arrays can be converted into table references with the UNNEST
keyword.</p>
<p>
<code class="literal">UNNEST(<array value expression>) [ WITH ORDINALITY
]</code>
</p>
<p>The <code class="literal"><array value expression></code> can be any
expression that evaluates to an array. A table is returned that contains
one column when WITH ORDINALITY is not used, or two columns when WITH
ORDINALITY is used. The first column contains the elements of the array
(including all the nulls). When the table has two columns, the second
column contains the ordinal position of the element in the array. When
UNNEST is used in the FROM clause of a query, it implies the LATERAL
keyword, which means the array that is converted to table can belong to
any table that precedes the UNNEST in the FROM clause. This is explained
in the <a class="link" href="dataaccess-chapt.html" title="Chapter 7. Data Access and Change">Data Access and Change</a> chapter.</p>
<p>
<span class="emphasis"><em>INLINE CONSTRUCTOR</em></span>
</p>
<p>Array constructors can be used in SELECT and other statements. For
example, an array constructor with a subquery can return the values from
several rows as one array.</p>
<p>The example below shows an ARRAY constructor with a correlated
subquery to return the list of order values for each customer. The
CUSTOMER table that is included for tests in the DatabaseManager GUI app
is the source of the data.</p>
<pre class="programlisting"> SELECT FIRSTNAME, LASTNAME, ARRAY(SELECT INVOICE.TOTAL FROM INVOICE WHERE CUSTOMERID = CUSTOMER.ID) AS ORDERS FROM CUSTOMER
FIRSTNAME LASTNAME ORDERS
--------- --------- --------------------------------------
Laura Steel ARRAY[2700.90,4235.70]
Robert King ARRAY[4761.60]
Robert Sommer ARRAY[]
Michael Smith ARRAY[3420.30]
</pre>
<p>
<span class="emphasis"><em>COMPARISON</em></span>
</p>
<p>Arrays can be compared for equality, but they cannot be compared
for ordering values or range comparison. Array expressions are therefore
not allowed in an ORDER BY clause, or in a comparison expression such as
GREATER THAN. It is possible to define a UNIQUE constraint on a column
of ARRAY type. Two arrays are equal if they have the same length and the
values at each index position are either equal or both NULL.</p>
<p>
<span class="emphasis"><em>USER DEFINED FUNCTIONS and PROCEDURES</em></span>
</p>
<p>Array parameters, variables and return values can be specified in
user defined functions and procedures, including aggregate functions. An
aggregate function can return an array that contains all the scalar
values that have been aggregated. These capabilities allow a wider range
of applications to be covered by user defined functions and easier data
exchange between the engine and the user's application.</p>
</div>
</div>
<div class="section" title="Indexes and Query Speed">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_index_speed"></a>Indexes and Query Speed</h2>
</div>
</div>
</div>
<p>HyperSQL supports PRIMARY KEY, UNIQUE and FOREIGN KEY constraints,
which can span multiple columns.</p>
<p>The engine creates indexes internally to support PRIMARY KEY, UNIQUE
and FOREIGN KEY constraints: a unique index is created for each PRIMARY
KEY or UNIQUE constraint; an ordinary index is created for each FOREIGN
KEY constraint.</p>
<p>HyperSQL allows defining indexes on single or multiple columns. You
should not create duplicate user-defined indexes on the same column sets
covered by constraints. This would result in unnecessary memory and speed
overheads. See the discussion in the <a class="link" href="deployment-chapt.html" title="Chapter 16. Deployment Guide">Deployment Guide</a> chapter for more
information.</p>
<p>Indexes are crucial for adequate query speed. When range or equality
conditions are used e.g. <code class="literal">SELECT ... WHERE acol > 10 AND bcol =
0</code>, an index should exist on one of the columns that has a
condition. In this example, the <code class="literal">bcol</code> column is the best
candidate. HyperSQL always uses the best condition and index. If there are
two indexes, one on acol, and another on bcol, it will choose the index on
bcol.</p>
<p>Queries always return results whether indexes exist or not, but they
return much faster when an index exists. As a rule of thumb, HSQLDB is
capable of internal processing of queries at over 100,000 rows per second.
Any query that runs into several seconds is clearly accessing thousands of
rows. The query should be checked and indexes should be added to the
relevant columns of the tables if necessary. The <code class="literal">EXPLAIN PLAN FOR
<query></code> statement can be used to see which indexes are
used to process the query.</p>
<p>When executing a DELETE or UPDATE statement, the engine needs to
find the rows that are to be deleted or updated. If there is an index on
one of the columns in the WHERE clause, it is often possible to start
directly from the first candidate row. Otherwise all the rows of the table
have to be examined.</p>
<p>Indexes are even more important in joins between multiple tables.
<code class="literal">SELECT ... FROM t1 JOIN t2 ON t1.c1 = t2.c2 </code> is
performed by taking rows of t1 one by one and finding a matching row in
t2. If there is no index on t2.c2 then for each row of t1, all the rows of
t2 must be checked. Whereas with an index, a matching row can be found in
a fraction of the time. If the query also has a condition on t1, e.g.,
<code class="literal">SELECT ... FROM t1 JOIN t2 ON t1.c1 = t2.c2 WHERE t1.c3 =
4</code> then an index on t1.c3 would eliminate the need for checking
all the rows of t1 one by one, and will reduce query time to less than a
millisecond per returned row. So if t1 and t2 each contain 10,000 rows,
the query without indexes involves checking 100,000,000 row combinations.
With an index on t2.c2, this is reduced to 10,000 row checks and index
lookups. With the additional index on t2.c2, only about 4 rows are checked
to get the first result row.</p>
<p>Note that in HSQLDB an index on multiple columns can be used
internally as a non-unique index on the first column in the list. For
example: <code class="literal">CONSTRAINT name1 UNIQUE (c1, c2, c3); </code> means
there is the equivalent of <code class="literal">CREATE INDEX name2 ON
atable(c1);</code>. So you do not need to specify an extra index if you
require one on the first column of the list.</p>
<p>In HyperSQL 2, a multi-column index will speed up queries that
contain joins or values on the first n columns of the index. You need NOT
declare additional individual indexes on those columns unless you use
queries that search only on a subset of the columns. For example, rows of
a table that has a PRIMARY KEY or UNIQUE constraint on three columns or
simply an ordinary index on those columns can be found efficiently when
values for all three columns, or the first two columns, or the first
column, are specified in the WHERE clause. For example, <code class="literal">SELECT
... FROM t1 WHERE t1.c1 = 4 AND t1.c2 = 6 AND t1.c3 = 8 </code>will use
an index on <code class="literal">t1(c1,c2,c3)</code> if it exists.</p>
<p>A multi-column index will not speed up queries on the second or
third column only. The first column must be specified in the JOIN .. ON or
WHERE conditions.</p>
<p>Sometimes query speed depends on the order of the tables in the JOIN
.. ON or FROM clauses. For example the second query below should be faster
with large tables (provided there is an index on
<code class="literal">TB.COL3</code>). The reason is that <code class="literal">TB.COL3</code>
can be evaluated very quickly if it applies to the first table (and there
is an index on <code class="literal">TB.COL3</code>):</p>
<div class="informalexample">
<pre class="programlisting"> -- TB is a very large table with only a few rows where TB.COL3 = 4
SELECT * FROM TA JOIN TB ON TA.COL1 = TB.COL2 AND TB.COL3 = 4;
SELECT * FROM TB JOIN TA ON TA.COL1 = TB.COL2 AND TB.COL3 = 4;</pre>
</div>
<p>The general rule is to put first the table that has a narrowing
condition on one of its columns. In certain cases, HyperSQL 2.2.x reorders
the joined tables if it is obvious that this will introduce a narrowing
condition.</p>
<p>HyperSQL features automatic, on-the-fly indexes for views and
subselects that are used in a query.</p>
<p>Indexes are used when a LIKE condition searches from the start of
the string.</p>
<p>Indexes are used for ORDER BY clauses if the same index is used for
selection and ordering of rows. It is possible to force the use of index
for ORDER BY.</p>
</div>
<div class="section" title="Query Processing and Optimisation">
<div class="titlepage">
<div>
<div>
<h2 class="title" style="clear: both">
<a name="sgc_query_opt"></a>Query Processing and Optimisation</h2>
</div>
</div>
</div>
<p>HyperSQL 2.3.x changes the order of tables in a query in order to
optimise processing. This happens only when one of the tables has a
narrowing condition and reordering does not change the result of the
query.</p>
<div class="section" title="Indexes and Conditions">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_indexes_cond"></a>Indexes and Conditions</h3>
</div>
</div>
</div>
<p>HyperSQL optimises queries to use indexes, for all types of range
and equality conditions, including IS NULL and NOT NULL conditions.
Conditions can be in join or WHERE clauses, including all types of
joins.</p>
<p>In addition, HyperSQL will use an index (if one exists) for IN
conditions, whether constants, variable, or subqueries are used on the
right hand side of the IN predicate. Multicolumn IN conditions can also
use an index.</p>
<p>HyperSQL can always use indexes when several conditions are
combined with the AND operator, choosing a conditions which can use an
index. This now extended to all equality conditions on multiple columns
that are part of an index.</p>
<p>HyperSQL will also use indexes when several conditions are
combined with the OR operator and each condition can use an index (each
condition may use a different index). For example, if a huge table has
two separate columns for first name and last name, and both columns are
indexed, a query such as the following example will use the indexes and
complete in a short time:</p>
<div class="informalexample">
<pre class="programlisting"> -- TC is a very large table
SELECT * FROM TC WHERE TC.FIRSTNAME = 'John' OR TC.LASTNAME = 'Smith' OR TC.LASTNAME = 'Williams'
</pre>
</div>
<p>Each subquery is considered a separate SELECT statement and uses
indexes when they are available.</p>
<p>In each SELECT statement, at least one index per table can be used
if there is a query conditions that can use the index. When conditions
on a table are combined with the OR operator, and each condition can use
an index, multiple indexes per table are used.</p>
</div>
<div class="section" title="Indexes and Operations">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_indexes_ops"></a>Indexes and Operations</h3>
</div>
</div>
</div>
<p>HyperSQL optimises simple row count queries in the form of
<code class="literal">SELECT COUNT(*) FROM <table></code> and returns the
result immediately (this optimisation does not take place in MVCC
mode).</p>
<p>HyperSQL can use an index on a column for <code class="literal">SELECT
MAX(<column>) FROM <table></code> and <code class="literal">SELECT
MIN(<column>) FROM <table></code> queries. There should
be an index on the <column> and the query can have a WHERE
condition on the same column. In the example below the maximum value for
the TB.COL3 below 1000000 is returned.</p>
<div class="informalexample">
<pre class="programlisting"> SELECT MAX(TB.COL3) FROM TB WHERE TB.COL < 1000000
</pre>
</div>
<p>HyperSQL can use an index for simple queries containing DISTINCT
or GROUP BY to avoid checking all the rows of the table. Note that
indexes are always used if the query has a condition, regardless of the
use of DISTINCT or GROUP BY. This particular optimisation applies to
cases in which all the columns in the SELECT list are from the same
table and are covered by a single index, and any join or query condition
uses this index.</p>
<p>For example, with the large table below, a DISTINCT or GROUP BY
query to return all the last names, can use an the index on the
TC.LASTNAME column. Similarly, a GROUP BY query on two columns can use
an index that covers the two columns.</p>
<pre class="programlisting"> -- TC is a very large table
SELECT DISTINCT LASTNAME FROM TC WHERE TC.LASTNAME > 'F'
SELECT STATE, LASTNAME FROM TC GROUP BY STATE, LASTNAME
</pre>
</div>
<div class="section" title="Indexes and ORDER BY, OFFSET and LIMIT">
<div class="titlepage">
<div>
<div>
<h3 class="title">
<a name="sgc_indexes_order"></a>Indexes and ORDER BY, OFFSET and LIMIT</h3>
</div>
</div>
</div>
<p>HyperSQL can use an index on an ORDER BY clause if all the columns
in ORDER BY are in a single-column or multi-column index (in the exact
order). This is important if there is a LIMIT n (or FETCH n ROWS ONLY)
clause. In this situation, the use of index allows the query processor
to access only the number of rows specified in the LIMIT clause, instead
of building the whole result set, which can be huge. This also works for
joined tables when the ORDER BY clause is on the columns of the first
table in a join. Indexes are used in the same way when ORDER BY ... DESC
is specified in the query. Note that unlike some other RDBMS, HyperSQL
does not need or create DESC indexes. It can use any ordinary, ascending
index for ORDER BY ... DESC.</p>
<p>If there is an equality or range condition (e.g. EQUALS, GREATER
THAN) condition on the columns specified in the ORDER BY clause, the
index is still used.</p>
<p>In the two examples below, the index on TA.COL3 is used and only
up to 1000 rows are processed and returned.</p>
<div class="informalexample">
<pre class="programlisting"> -- TA is a very large table with an index on TA.COL3
SELECT * FROM TA JOIN TB ON TA.COL2 = TB.COL1 WHERE TA.COL3 > 40000 ORDER BY TA.COL3 LIMIT 1000;
SELECT * FROM TA JOIN TB ON TA.COL2 = TB.COL1 WHERE TA.COL3 > 40000 AND TA.COL3 < 100000 ORDER BY TA.COL3 DESC LIMIT 1000;
</pre>
</div>
<p>But if the query contains an equality condition on another indexed
column in the table, this may take precedence and no index may be used
for ORDER BY. In this case USING INDEX can be added to the end of the
query to force the use of the index for the LIMIT operation. In the
example below there is an index on TA.COL1 as well as the index on
TA.COL3. Normally the index on TA.COL1 is used, but the USING INDEX hint
results in the index on TB.COL3 to be used for selecting the first 1000
rows.</p>
<div class="informalexample">
<pre class="programlisting"> -- TA is a very large table with an index on TA.COL3 and a separate index on TA.COL1
SELECT * FROM TA JOIN TB ON TA.COL2 = TB.COL1 WHERE TA.COL1 = 'SENT' AND TB.COL3 > 40000 ORDER BY TB.COL3 LIMIT 1000 USING INDEX;
</pre>
</div>
<p></p>
</div>
</div>
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