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><H1
><A
NAME="SQL-SELECT"
></A
>SELECT</H1
><DIV
CLASS="REFNAMEDIV"
><A
NAME="AEN59451"
></A
><H2
>Name</H2
>SELECT -- retrieve rows from a table or view</DIV
><A
NAME="AEN59454"
></A
><DIV
CLASS="REFSYNOPSISDIV"
><A
NAME="AEN59456"
></A
><H2
>Synopsis</H2
><PRE
CLASS="SYNOPSIS"
>SELECT [ ALL | DISTINCT [ ON ( <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [, ...] ) ] ]
* | <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [ AS <TT
CLASS="REPLACEABLE"
><I
>output_name</I
></TT
> ] [, ...]
[ FROM <TT
CLASS="REPLACEABLE"
><I
>from_item</I
></TT
> [, ...] ]
[ WHERE <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> ]
[ GROUP BY <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [, ...] ]
[ HAVING <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> [, ...] ]
[ { UNION | INTERSECT | EXCEPT } [ ALL ] <TT
CLASS="REPLACEABLE"
><I
>select</I
></TT
> ]
[ ORDER BY <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [ ASC | DESC | USING <TT
CLASS="REPLACEABLE"
><I
>operator</I
></TT
> ] [ NULLS { FIRST | LAST } ] [, ...] ]
[ LIMIT { <TT
CLASS="REPLACEABLE"
><I
>count</I
></TT
> | ALL } ]
[ OFFSET <TT
CLASS="REPLACEABLE"
><I
>start</I
></TT
> ]
[ FOR { UPDATE | SHARE } [ OF <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
> [, ...] ] [ NOWAIT ] [...] ]
where <TT
CLASS="REPLACEABLE"
><I
>from_item</I
></TT
> can be one of:
[ ONLY ] <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
> [ * ] [ [ AS ] <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
> [ ( <TT
CLASS="REPLACEABLE"
><I
>column_alias</I
></TT
> [, ...] ) ] ]
( <TT
CLASS="REPLACEABLE"
><I
>select</I
></TT
> ) [ AS ] <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
> [ ( <TT
CLASS="REPLACEABLE"
><I
>column_alias</I
></TT
> [, ...] ) ]
<TT
CLASS="REPLACEABLE"
><I
>function_name</I
></TT
> ( [ <TT
CLASS="REPLACEABLE"
><I
>argument</I
></TT
> [, ...] ] ) [ AS ] <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
> [ ( <TT
CLASS="REPLACEABLE"
><I
>column_alias</I
></TT
> [, ...] | <TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> [, ...] ) ]
<TT
CLASS="REPLACEABLE"
><I
>function_name</I
></TT
> ( [ <TT
CLASS="REPLACEABLE"
><I
>argument</I
></TT
> [, ...] ] ) AS ( <TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> [, ...] )
<TT
CLASS="REPLACEABLE"
><I
>from_item</I
></TT
> [ NATURAL ] <TT
CLASS="REPLACEABLE"
><I
>join_type</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>from_item</I
></TT
> [ ON <TT
CLASS="REPLACEABLE"
><I
>join_condition</I
></TT
> | USING ( <TT
CLASS="REPLACEABLE"
><I
>join_column</I
></TT
> [, ...] ) ]</PRE
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN59491"
></A
><H2
>Description</H2
><P
> <TT
CLASS="COMMAND"
>SELECT</TT
> retrieves rows from zero or more tables.
The general processing of <TT
CLASS="COMMAND"
>SELECT</TT
> is as follows:
<P
></P
></P><OL
TYPE="1"
><LI
><P
> All elements in the <TT
CLASS="LITERAL"
>FROM</TT
> list are computed.
(Each element in the <TT
CLASS="LITERAL"
>FROM</TT
> list is a real or
virtual table.) If more than one element is specified in the
<TT
CLASS="LITERAL"
>FROM</TT
> list, they are cross-joined together.
(See <A
HREF="sql-select.html#SQL-FROM"
><I
><I
>FROM</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> If the <TT
CLASS="LITERAL"
>WHERE</TT
> clause is specified, all rows
that do not satisfy the condition are eliminated from the
output. (See <A
HREF="sql-select.html#SQL-WHERE"
><I
><I
>WHERE</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> If the <TT
CLASS="LITERAL"
>GROUP BY</TT
> clause is specified, the
output is divided into groups of rows that match on one or more
values. If the <TT
CLASS="LITERAL"
>HAVING</TT
> clause is present, it
eliminates groups that do not satisfy the given condition. (See
<A
HREF="sql-select.html#SQL-GROUPBY"
><I
><I
>GROUP BY</I
> Clause</I
></A
> and
<A
HREF="sql-select.html#SQL-HAVING"
><I
><I
>HAVING</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> The actual output rows are computed using the
<TT
CLASS="COMMAND"
>SELECT</TT
> output expressions for each selected
row. (See
<A
HREF="sql-select.html#SQL-SELECT-LIST"
><I
><I
>SELECT</I
> List</I
></A
>
below.)
</P
></LI
><LI
><P
> Using the operators <TT
CLASS="LITERAL"
>UNION</TT
>,
<TT
CLASS="LITERAL"
>INTERSECT</TT
>, and <TT
CLASS="LITERAL"
>EXCEPT</TT
>, the
output of more than one <TT
CLASS="COMMAND"
>SELECT</TT
> statement can
be combined to form a single result set. The
<TT
CLASS="LITERAL"
>UNION</TT
> operator returns all rows that are in
one or both of the result sets. The
<TT
CLASS="LITERAL"
>INTERSECT</TT
> operator returns all rows that are
strictly in both result sets. The <TT
CLASS="LITERAL"
>EXCEPT</TT
>
operator returns the rows that are in the first result set but
not in the second. In all three cases, duplicate rows are
eliminated unless <TT
CLASS="LITERAL"
>ALL</TT
> is specified. (See
<A
HREF="sql-select.html#SQL-UNION"
><I
><I
>UNION</I
> Clause</I
></A
>, <A
HREF="sql-select.html#SQL-INTERSECT"
><I
><I
>INTERSECT</I
> Clause</I
></A
>, and
<A
HREF="sql-select.html#SQL-EXCEPT"
><I
><I
>EXCEPT</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> If the <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause is specified, the
returned rows are sorted in the specified order. If
<TT
CLASS="LITERAL"
>ORDER BY</TT
> is not given, the rows are returned
in whatever order the system finds fastest to produce. (See
<A
HREF="sql-select.html#SQL-ORDERBY"
><I
><I
>ORDER BY</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> <TT
CLASS="LITERAL"
>DISTINCT</TT
> eliminates duplicate rows from the
result. <TT
CLASS="LITERAL"
>DISTINCT ON</TT
> eliminates rows that
match on all the specified expressions. <TT
CLASS="LITERAL"
>ALL</TT
>
(the default) will return all candidate rows, including
duplicates. (See <A
HREF="sql-select.html#SQL-DISTINCT"
><I
><I
>DISTINCT</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> If the <TT
CLASS="LITERAL"
>LIMIT</TT
> or <TT
CLASS="LITERAL"
>OFFSET</TT
>
clause is specified, the <TT
CLASS="COMMAND"
>SELECT</TT
> statement
only returns a subset of the result rows. (See <A
HREF="sql-select.html#SQL-LIMIT"
><I
><I
>LIMIT</I
> Clause</I
></A
> below.)
</P
></LI
><LI
><P
> If <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
>
is specified, the
<TT
CLASS="COMMAND"
>SELECT</TT
> statement locks the selected rows
against concurrent updates. (See <A
HREF="sql-select.html#SQL-FOR-UPDATE-SHARE"
><I
><I
>FOR UPDATE</I
>/<I
>FOR SHARE</I
> Clause</I
></A
> below.)
</P
></LI
></OL
><P>
</P
><P
> You must have <TT
CLASS="LITERAL"
>SELECT</TT
> privilege on a table to
read its values. The use of <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or
<TT
CLASS="LITERAL"
>FOR SHARE</TT
> requires
<TT
CLASS="LITERAL"
>UPDATE</TT
> privilege as well.
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN59558"
></A
><H2
>Parameters</H2
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-FROM"
></A
><H3
><TT
CLASS="LITERAL"
>FROM</TT
> Clause</H3
><P
> The <TT
CLASS="LITERAL"
>FROM</TT
> clause specifies one or more source
tables for the <TT
CLASS="COMMAND"
>SELECT</TT
>. If multiple sources are
specified, the result is the Cartesian product (cross join) of all
the sources. But usually qualification conditions
are added to restrict the returned rows to a small subset of the
Cartesian product.
</P
><P
> The <TT
CLASS="LITERAL"
>FROM</TT
> clause can contain the following
elements:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
></DT
><DD
><P
> The name (optionally schema-qualified) of an existing table or
view. If <TT
CLASS="LITERAL"
>ONLY</TT
> is specified, only that table is
scanned. If <TT
CLASS="LITERAL"
>ONLY</TT
> is not specified, the table and
all its descendant tables (if any) are scanned. <TT
CLASS="LITERAL"
>*</TT
>
can be appended to the table name to indicate that descendant
tables are to be scanned, but in the current version, this is
the default behavior. (In releases before 7.1,
<TT
CLASS="LITERAL"
>ONLY</TT
> was the default behavior.) The default
behavior can be modified by changing the <A
HREF="runtime-config-compatible.html#GUC-SQL-INHERITANCE"
>sql_inheritance</A
> configuration option.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
></DT
><DD
><P
> A substitute name for the <TT
CLASS="LITERAL"
>FROM</TT
> item containing the
alias. An alias is used for brevity or to eliminate ambiguity
for self-joins (where the same table is scanned multiple
times). When an alias is provided, it completely hides the
actual name of the table or function; for example given
<TT
CLASS="LITERAL"
>FROM foo AS f</TT
>, the remainder of the
<TT
CLASS="COMMAND"
>SELECT</TT
> must refer to this <TT
CLASS="LITERAL"
>FROM</TT
>
item as <TT
CLASS="LITERAL"
>f</TT
> not <TT
CLASS="LITERAL"
>foo</TT
>. If an alias is
written, a column alias list can also be written to provide
substitute names for one or more columns of the table.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>select</I
></TT
></DT
><DD
><P
> A sub-<TT
CLASS="COMMAND"
>SELECT</TT
> can appear in the
<TT
CLASS="LITERAL"
>FROM</TT
> clause. This acts as though its
output were created as a temporary table for the duration of
this single <TT
CLASS="COMMAND"
>SELECT</TT
> command. Note that the
sub-<TT
CLASS="COMMAND"
>SELECT</TT
> must be surrounded by
parentheses, and an alias <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
> be
provided for it. A
<A
HREF="sql-values.html"
><I
>VALUES</I
></A
> command
can also be used here.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>function_name</I
></TT
></DT
><DD
><P
> Function calls can appear in the <TT
CLASS="LITERAL"
>FROM</TT
>
clause. (This is especially useful for functions that return
result sets, but any function can be used.) This acts as
though its output were created as a temporary table for the
duration of this single <TT
CLASS="COMMAND"
>SELECT</TT
> command. An
alias can also be used. If an alias is written, a column alias
list can also be written to provide substitute names for one
or more attributes of the function's composite return type. If
the function has been defined as returning the <TT
CLASS="TYPE"
>record</TT
>
data type, then an alias or the key word <TT
CLASS="LITERAL"
>AS</TT
> must
be present, followed by a column definition list in the form
<TT
CLASS="LITERAL"
>( <TT
CLASS="REPLACEABLE"
><I
>column_name</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>data_type</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>]
)</TT
>. The column definition list must match the actual
number and types of columns returned by the function.
</P
></DD
><DT
><TT
CLASS="REPLACEABLE"
><I
>join_type</I
></TT
></DT
><DD
><P
> One of
<P
></P
></P><UL
><LI
><P
><TT
CLASS="LITERAL"
>[ INNER ] JOIN</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>LEFT [ OUTER ] JOIN</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>RIGHT [ OUTER ] JOIN</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>FULL [ OUTER ] JOIN</TT
></P
></LI
><LI
><P
><TT
CLASS="LITERAL"
>CROSS JOIN</TT
></P
></LI
></UL
><P>
For the <TT
CLASS="LITERAL"
>INNER</TT
> and <TT
CLASS="LITERAL"
>OUTER</TT
> join types, a
join condition must be specified, namely exactly one of
<TT
CLASS="LITERAL"
>NATURAL</TT
>, <TT
CLASS="LITERAL"
>ON <TT
CLASS="REPLACEABLE"
><I
>join_condition</I
></TT
></TT
>, or
<TT
CLASS="LITERAL"
>USING (<TT
CLASS="REPLACEABLE"
><I
>join_column</I
></TT
> [, ...])</TT
>.
See below for the meaning. For <TT
CLASS="LITERAL"
>CROSS JOIN</TT
>,
none of these clauses can appear.
</P
><P
> A <TT
CLASS="LITERAL"
>JOIN</TT
> clause combines two
<TT
CLASS="LITERAL"
>FROM</TT
> items. Use parentheses if necessary to
determine the order of nesting. In the absence of parentheses,
<TT
CLASS="LITERAL"
>JOIN</TT
>s nest left-to-right. In any case
<TT
CLASS="LITERAL"
>JOIN</TT
> binds more tightly than the commas
separating <TT
CLASS="LITERAL"
>FROM</TT
> items.
</P
><P
> <TT
CLASS="LITERAL"
>CROSS JOIN</TT
> and <TT
CLASS="LITERAL"
>INNER JOIN</TT
>
produce a simple Cartesian product, the same result as you get from
listing the two items at the top level of <TT
CLASS="LITERAL"
>FROM</TT
>,
but restricted by the join condition (if any).
<TT
CLASS="LITERAL"
>CROSS JOIN</TT
> is equivalent to <TT
CLASS="LITERAL"
>INNER JOIN ON
(TRUE)</TT
>, that is, no rows are removed by qualification.
These join types are just a notational convenience, since they
do nothing you couldn't do with plain <TT
CLASS="LITERAL"
>FROM</TT
> and
<TT
CLASS="LITERAL"
>WHERE</TT
>.
</P
><P
> <TT
CLASS="LITERAL"
>LEFT OUTER JOIN</TT
> returns all rows in the qualified
Cartesian product (i.e., all combined rows that pass its join
condition), plus one copy of each row in the left-hand table
for which there was no right-hand row that passed the join
condition. This left-hand row is extended to the full width
of the joined table by inserting null values for the
right-hand columns. Note that only the <TT
CLASS="LITERAL"
>JOIN</TT
>
clause's own condition is considered while deciding which rows
have matches. Outer conditions are applied afterwards.
</P
><P
> Conversely, <TT
CLASS="LITERAL"
>RIGHT OUTER JOIN</TT
> returns all the
joined rows, plus one row for each unmatched right-hand row
(extended with nulls on the left). This is just a notational
convenience, since you could convert it to a <TT
CLASS="LITERAL"
>LEFT
OUTER JOIN</TT
> by switching the left and right inputs.
</P
><P
> <TT
CLASS="LITERAL"
>FULL OUTER JOIN</TT
> returns all the joined rows, plus
one row for each unmatched left-hand row (extended with nulls
on the right), plus one row for each unmatched right-hand row
(extended with nulls on the left).
</P
></DD
><DT
><TT
CLASS="LITERAL"
>ON <TT
CLASS="REPLACEABLE"
><I
>join_condition</I
></TT
></TT
></DT
><DD
><P
> <TT
CLASS="REPLACEABLE"
><I
>join_condition</I
></TT
> is
an expression resulting in a value of type
<TT
CLASS="TYPE"
>boolean</TT
> (similar to a <TT
CLASS="LITERAL"
>WHERE</TT
>
clause) that specifies which rows in a join are considered to
match.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>USING ( <TT
CLASS="REPLACEABLE"
><I
>join_column</I
></TT
> [, ...] )</TT
></DT
><DD
><P
> A clause of the form <TT
CLASS="LITERAL"
>USING ( a, b, ... )</TT
> is
shorthand for <TT
CLASS="LITERAL"
>ON left_table.a = right_table.a AND
left_table.b = right_table.b ...</TT
>. Also,
<TT
CLASS="LITERAL"
>USING</TT
> implies that only one of each pair of
equivalent columns will be included in the join output, not
both.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>NATURAL</TT
></DT
><DD
><P
> <TT
CLASS="LITERAL"
>NATURAL</TT
> is shorthand for a
<TT
CLASS="LITERAL"
>USING</TT
> list that mentions all columns in the two
tables that have the same names.
</P
></DD
></DL
></DIV
><P>
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-WHERE"
></A
><H3
><TT
CLASS="LITERAL"
>WHERE</TT
> Clause</H3
><P
> The optional <TT
CLASS="LITERAL"
>WHERE</TT
> clause has the general form
</P><PRE
CLASS="SYNOPSIS"
>WHERE <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
></PRE
><P>
where <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> is
any expression that evaluates to a result of type
<TT
CLASS="TYPE"
>boolean</TT
>. Any row that does not satisfy this
condition will be eliminated from the output. A row satisfies the
condition if it returns true when the actual row values are
substituted for any variable references.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-GROUPBY"
></A
><H3
><TT
CLASS="LITERAL"
>GROUP BY</TT
> Clause</H3
><P
> The optional <TT
CLASS="LITERAL"
>GROUP BY</TT
> clause has the general form
</P><PRE
CLASS="SYNOPSIS"
>GROUP BY <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [, ...]</PRE
><P>
</P
><P
> <TT
CLASS="LITERAL"
>GROUP BY</TT
> will condense into a single row all
selected rows that share the same values for the grouped
expressions. <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> can be an input column
name, or the name or ordinal number of an output column
(<TT
CLASS="COMMAND"
>SELECT</TT
> list item), or an arbitrary
expression formed from input-column values. In case of ambiguity,
a <TT
CLASS="LITERAL"
>GROUP BY</TT
> name will be interpreted as an
input-column name rather than an output column name.
</P
><P
> Aggregate functions, if any are used, are computed across all rows
making up each group, producing a separate value for each group
(whereas without <TT
CLASS="LITERAL"
>GROUP BY</TT
>, an aggregate
produces a single value computed across all the selected rows).
When <TT
CLASS="LITERAL"
>GROUP BY</TT
> is present, it is not valid for
the <TT
CLASS="COMMAND"
>SELECT</TT
> list expressions to refer to
ungrouped columns except within aggregate functions, since there
would be more than one possible value to return for an ungrouped
column.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-HAVING"
></A
><H3
><TT
CLASS="LITERAL"
>HAVING</TT
> Clause</H3
><P
> The optional <TT
CLASS="LITERAL"
>HAVING</TT
> clause has the general form
</P><PRE
CLASS="SYNOPSIS"
>HAVING <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
></PRE
><P>
where <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> is
the same as specified for the <TT
CLASS="LITERAL"
>WHERE</TT
> clause.
</P
><P
> <TT
CLASS="LITERAL"
>HAVING</TT
> eliminates group rows that do not
satisfy the condition. <TT
CLASS="LITERAL"
>HAVING</TT
> is different
from <TT
CLASS="LITERAL"
>WHERE</TT
>: <TT
CLASS="LITERAL"
>WHERE</TT
> filters
individual rows before the application of <TT
CLASS="LITERAL"
>GROUP
BY</TT
>, while <TT
CLASS="LITERAL"
>HAVING</TT
> filters group rows
created by <TT
CLASS="LITERAL"
>GROUP BY</TT
>. Each column referenced in
<TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> must
unambiguously reference a grouping column, unless the reference
appears within an aggregate function.
</P
><P
> The presence of <TT
CLASS="LITERAL"
>HAVING</TT
> turns a query into a grouped
query even if there is no <TT
CLASS="LITERAL"
>GROUP BY</TT
> clause. This is the
same as what happens when the query contains aggregate functions but
no <TT
CLASS="LITERAL"
>GROUP BY</TT
> clause. All the selected rows are considered to
form a single group, and the <TT
CLASS="COMMAND"
>SELECT</TT
> list and
<TT
CLASS="LITERAL"
>HAVING</TT
> clause can only reference table columns from
within aggregate functions. Such a query will emit a single row if the
<TT
CLASS="LITERAL"
>HAVING</TT
> condition is true, zero rows if it is not true.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-SELECT-LIST"
></A
><H3
><TT
CLASS="COMMAND"
>SELECT</TT
> List</H3
><P
> The <TT
CLASS="COMMAND"
>SELECT</TT
> list (between the key words
<TT
CLASS="LITERAL"
>SELECT</TT
> and <TT
CLASS="LITERAL"
>FROM</TT
>) specifies expressions
that form the output rows of the <TT
CLASS="COMMAND"
>SELECT</TT
>
statement. The expressions can (and usually do) refer to columns
computed in the <TT
CLASS="LITERAL"
>FROM</TT
> clause. Using the clause
<TT
CLASS="LITERAL"
>AS <TT
CLASS="REPLACEABLE"
><I
>output_name</I
></TT
></TT
>, another
name can be specified for an output column. This name is
primarily used to label the column for display. It can also be
used to refer to the column's value in <TT
CLASS="LITERAL"
>ORDER BY</TT
> and
<TT
CLASS="LITERAL"
>GROUP BY</TT
> clauses, but not in the <TT
CLASS="LITERAL"
>WHERE</TT
> or
<TT
CLASS="LITERAL"
>HAVING</TT
> clauses; there you must write out the
expression instead.
</P
><P
> Instead of an expression, <TT
CLASS="LITERAL"
>*</TT
> can be written in
the output list as a shorthand for all the columns of the selected
rows. Also, one can write <TT
CLASS="LITERAL"
><TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
>.*</TT
> as a
shorthand for the columns coming from just that table.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-UNION"
></A
><H3
><TT
CLASS="LITERAL"
>UNION</TT
> Clause</H3
><P
> The <TT
CLASS="LITERAL"
>UNION</TT
> clause has this general form:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
> UNION [ ALL ] <TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
></PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
> is
any <TT
CLASS="COMMAND"
>SELECT</TT
> statement without an <TT
CLASS="LITERAL"
>ORDER
BY</TT
>, <TT
CLASS="LITERAL"
>LIMIT</TT
>, <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>, or
<TT
CLASS="LITERAL"
>FOR SHARE</TT
> clause.
(<TT
CLASS="LITERAL"
>ORDER BY</TT
> and <TT
CLASS="LITERAL"
>LIMIT</TT
> can be attached to a
subexpression if it is enclosed in parentheses. Without
parentheses, these clauses will be taken to apply to the result of
the <TT
CLASS="LITERAL"
>UNION</TT
>, not to its right-hand input
expression.)
</P
><P
> The <TT
CLASS="LITERAL"
>UNION</TT
> operator computes the set union of
the rows returned by the involved <TT
CLASS="COMMAND"
>SELECT</TT
>
statements. A row is in the set union of two result sets if it
appears in at least one of the result sets. The two
<TT
CLASS="COMMAND"
>SELECT</TT
> statements that represent the direct
operands of the <TT
CLASS="LITERAL"
>UNION</TT
> must produce the same
number of columns, and corresponding columns must be of compatible
data types.
</P
><P
> The result of <TT
CLASS="LITERAL"
>UNION</TT
> does not contain any duplicate
rows unless the <TT
CLASS="LITERAL"
>ALL</TT
> option is specified.
<TT
CLASS="LITERAL"
>ALL</TT
> prevents elimination of duplicates. (Therefore,
<TT
CLASS="LITERAL"
>UNION ALL</TT
> is usually significantly quicker than
<TT
CLASS="LITERAL"
>UNION</TT
>; use <TT
CLASS="LITERAL"
>ALL</TT
> when you can.)
</P
><P
> Multiple <TT
CLASS="LITERAL"
>UNION</TT
> operators in the same
<TT
CLASS="COMMAND"
>SELECT</TT
> statement are evaluated left to right,
unless otherwise indicated by parentheses.
</P
><P
> Currently, <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> and <TT
CLASS="LITERAL"
>FOR SHARE</TT
> cannot be
specified either for a <TT
CLASS="LITERAL"
>UNION</TT
> result or for any input of a
<TT
CLASS="LITERAL"
>UNION</TT
>.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-INTERSECT"
></A
><H3
><TT
CLASS="LITERAL"
>INTERSECT</TT
> Clause</H3
><P
> The <TT
CLASS="LITERAL"
>INTERSECT</TT
> clause has this general form:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
> INTERSECT [ ALL ] <TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
></PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
> is
any <TT
CLASS="COMMAND"
>SELECT</TT
> statement without an <TT
CLASS="LITERAL"
>ORDER
BY</TT
>, <TT
CLASS="LITERAL"
>LIMIT</TT
>, <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>, or
<TT
CLASS="LITERAL"
>FOR SHARE</TT
> clause.
</P
><P
> The <TT
CLASS="LITERAL"
>INTERSECT</TT
> operator computes the set
intersection of the rows returned by the involved
<TT
CLASS="COMMAND"
>SELECT</TT
> statements. A row is in the
intersection of two result sets if it appears in both result sets.
</P
><P
> The result of <TT
CLASS="LITERAL"
>INTERSECT</TT
> does not contain any
duplicate rows unless the <TT
CLASS="LITERAL"
>ALL</TT
> option is specified.
With <TT
CLASS="LITERAL"
>ALL</TT
>, a row that has <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> duplicates in the
left table and <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> duplicates in the right table will appear
min(<TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
>,<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>) times in the result set.
</P
><P
> Multiple <TT
CLASS="LITERAL"
>INTERSECT</TT
> operators in the same
<TT
CLASS="COMMAND"
>SELECT</TT
> statement are evaluated left to right,
unless parentheses dictate otherwise.
<TT
CLASS="LITERAL"
>INTERSECT</TT
> binds more tightly than
<TT
CLASS="LITERAL"
>UNION</TT
>. That is, <TT
CLASS="LITERAL"
>A UNION B INTERSECT
C</TT
> will be read as <TT
CLASS="LITERAL"
>A UNION (B INTERSECT
C)</TT
>.
</P
><P
> Currently, <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> and <TT
CLASS="LITERAL"
>FOR SHARE</TT
> cannot be
specified either for an <TT
CLASS="LITERAL"
>INTERSECT</TT
> result or for any input of
an <TT
CLASS="LITERAL"
>INTERSECT</TT
>.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-EXCEPT"
></A
><H3
><TT
CLASS="LITERAL"
>EXCEPT</TT
> Clause</H3
><P
> The <TT
CLASS="LITERAL"
>EXCEPT</TT
> clause has this general form:
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
> EXCEPT [ ALL ] <TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
></PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>select_statement</I
></TT
> is
any <TT
CLASS="COMMAND"
>SELECT</TT
> statement without an <TT
CLASS="LITERAL"
>ORDER
BY</TT
>, <TT
CLASS="LITERAL"
>LIMIT</TT
>, <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>, or
<TT
CLASS="LITERAL"
>FOR SHARE</TT
> clause.
</P
><P
> The <TT
CLASS="LITERAL"
>EXCEPT</TT
> operator computes the set of rows
that are in the result of the left <TT
CLASS="COMMAND"
>SELECT</TT
>
statement but not in the result of the right one.
</P
><P
> The result of <TT
CLASS="LITERAL"
>EXCEPT</TT
> does not contain any
duplicate rows unless the <TT
CLASS="LITERAL"
>ALL</TT
> option is specified.
With <TT
CLASS="LITERAL"
>ALL</TT
>, a row that has <TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
> duplicates in the
left table and <TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
> duplicates in the right table will appear
max(<TT
CLASS="REPLACEABLE"
><I
>m</I
></TT
>-<TT
CLASS="REPLACEABLE"
><I
>n</I
></TT
>,0) times in the result set.
</P
><P
> Multiple <TT
CLASS="LITERAL"
>EXCEPT</TT
> operators in the same
<TT
CLASS="COMMAND"
>SELECT</TT
> statement are evaluated left to right,
unless parentheses dictate otherwise. <TT
CLASS="LITERAL"
>EXCEPT</TT
> binds at
the same level as <TT
CLASS="LITERAL"
>UNION</TT
>.
</P
><P
> Currently, <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> and <TT
CLASS="LITERAL"
>FOR SHARE</TT
> cannot be
specified either for an <TT
CLASS="LITERAL"
>EXCEPT</TT
> result or for any input of
an <TT
CLASS="LITERAL"
>EXCEPT</TT
>.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-ORDERBY"
></A
><H3
><TT
CLASS="LITERAL"
>ORDER BY</TT
> Clause</H3
><P
> The optional <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause has this general form:
</P><PRE
CLASS="SYNOPSIS"
>ORDER BY <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [ ASC | DESC | USING <TT
CLASS="REPLACEABLE"
><I
>operator</I
></TT
> ] [ NULLS { FIRST | LAST } ] [, ...]</PRE
><P>
The <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause causes the result rows to
be sorted according to the specified expression(s). If two rows are
equal according to the leftmost expression, they are compared
according to the next expression and so on. If they are equal
according to all specified expressions, they are returned in
an implementation-dependent order.
</P
><P
> Each <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> can be the
name or ordinal number of an output column
(<TT
CLASS="COMMAND"
>SELECT</TT
> list item), or it can be an arbitrary
expression formed from input-column values.
</P
><P
> The ordinal number refers to the ordinal (left-to-right) position
of the result column. This feature makes it possible to define an
ordering on the basis of a column that does not have a unique
name. This is never absolutely necessary because it is always
possible to assign a name to a result column using the
<TT
CLASS="LITERAL"
>AS</TT
> clause.
</P
><P
> It is also possible to use arbitrary expressions in the
<TT
CLASS="LITERAL"
>ORDER BY</TT
> clause, including columns that do not
appear in the <TT
CLASS="COMMAND"
>SELECT</TT
> result list. Thus the
following statement is valid:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT name FROM distributors ORDER BY code;</PRE
><P>
A limitation of this feature is that an <TT
CLASS="LITERAL"
>ORDER BY</TT
>
clause applying to the result of a <TT
CLASS="LITERAL"
>UNION</TT
>,
<TT
CLASS="LITERAL"
>INTERSECT</TT
>, or <TT
CLASS="LITERAL"
>EXCEPT</TT
> clause can only
specify an output column name or number, not an expression.
</P
><P
> If an <TT
CLASS="LITERAL"
>ORDER BY</TT
> expression is a simple name that
matches both a result column name and an input column name,
<TT
CLASS="LITERAL"
>ORDER BY</TT
> will interpret it as the result column name.
This is the opposite of the choice that <TT
CLASS="LITERAL"
>GROUP BY</TT
> will
make in the same situation. This inconsistency is made to be
compatible with the SQL standard.
</P
><P
> Optionally one can add the key word <TT
CLASS="LITERAL"
>ASC</TT
> (ascending) or
<TT
CLASS="LITERAL"
>DESC</TT
> (descending) after any expression in the
<TT
CLASS="LITERAL"
>ORDER BY</TT
> clause. If not specified, <TT
CLASS="LITERAL"
>ASC</TT
> is
assumed by default. Alternatively, a specific ordering operator
name can be specified in the <TT
CLASS="LITERAL"
>USING</TT
> clause.
An ordering operator must be a less-than or greater-than
member of some B-tree operator family.
<TT
CLASS="LITERAL"
>ASC</TT
> is usually equivalent to <TT
CLASS="LITERAL"
>USING <</TT
> and
<TT
CLASS="LITERAL"
>DESC</TT
> is usually equivalent to <TT
CLASS="LITERAL"
>USING ></TT
>.
(But the creator of a user-defined data type can define exactly what the
default sort ordering is, and it might correspond to operators with other
names.)
</P
><P
> If <TT
CLASS="LITERAL"
>NULLS LAST</TT
> is specified, null values sort after all
non-null values; if <TT
CLASS="LITERAL"
>NULLS FIRST</TT
> is specified, null values
sort before all non-null values. If neither is specified, the default
behavior is <TT
CLASS="LITERAL"
>NULLS LAST</TT
> when <TT
CLASS="LITERAL"
>ASC</TT
> is specified
or implied, and <TT
CLASS="LITERAL"
>NULLS FIRST</TT
> when <TT
CLASS="LITERAL"
>DESC</TT
> is specified
(thus, the default is to act as though nulls are larger than non-nulls).
When <TT
CLASS="LITERAL"
>USING</TT
> is specified, the default nulls ordering depends
on whether the operator is a less-than or greater-than operator.
</P
><P
> Note that ordering options apply only to the expression they follow;
for example <TT
CLASS="LITERAL"
>ORDER BY x, y DESC</TT
> does not mean
the same thing as <TT
CLASS="LITERAL"
>ORDER BY x DESC, y DESC</TT
>.
</P
><P
> Character-string data is sorted according to the locale-specific
collation order that was established when the database cluster
was initialized.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-DISTINCT"
></A
><H3
><TT
CLASS="LITERAL"
>DISTINCT</TT
> Clause</H3
><P
> If <TT
CLASS="LITERAL"
>DISTINCT</TT
> is specified, all duplicate rows are
removed from the result set (one row is kept from each group of
duplicates). <TT
CLASS="LITERAL"
>ALL</TT
> specifies the opposite: all rows are
kept; that is the default.
</P
><P
> <TT
CLASS="LITERAL"
>DISTINCT ON ( <TT
CLASS="REPLACEABLE"
><I
>expression</I
></TT
> [, ...] )</TT
>
keeps only the first row of each set of rows where the given
expressions evaluate to equal. The <TT
CLASS="LITERAL"
>DISTINCT ON</TT
>
expressions are interpreted using the same rules as for
<TT
CLASS="LITERAL"
>ORDER BY</TT
> (see above). Note that the <SPAN
CLASS="QUOTE"
>"first
row"</SPAN
> of each set is unpredictable unless <TT
CLASS="LITERAL"
>ORDER
BY</TT
> is used to ensure that the desired row appears first. For
example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT DISTINCT ON (location) location, time, report
FROM weather_reports
ORDER BY location, time DESC;</PRE
><P>
retrieves the most recent weather report for each location. But
if we had not used <TT
CLASS="LITERAL"
>ORDER BY</TT
> to force descending order
of time values for each location, we'd have gotten a report from
an unpredictable time for each location.
</P
><P
> The <TT
CLASS="LITERAL"
>DISTINCT ON</TT
> expression(s) must match the leftmost
<TT
CLASS="LITERAL"
>ORDER BY</TT
> expression(s). The <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause
will normally contain additional expression(s) that determine the
desired precedence of rows within each <TT
CLASS="LITERAL"
>DISTINCT ON</TT
> group.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-LIMIT"
></A
><H3
><TT
CLASS="LITERAL"
>LIMIT</TT
> Clause</H3
><P
> The <TT
CLASS="LITERAL"
>LIMIT</TT
> clause consists of two independent
sub-clauses:
</P><PRE
CLASS="SYNOPSIS"
>LIMIT { <TT
CLASS="REPLACEABLE"
><I
>count</I
></TT
> | ALL }
OFFSET <TT
CLASS="REPLACEABLE"
><I
>start</I
></TT
></PRE
><P>
<TT
CLASS="REPLACEABLE"
><I
>count</I
></TT
> specifies the
maximum number of rows to return, while <TT
CLASS="REPLACEABLE"
><I
>start</I
></TT
> specifies the number of rows
to skip before starting to return rows. When both are specified,
<TT
CLASS="REPLACEABLE"
><I
>start</I
></TT
> rows are skipped
before starting to count the <TT
CLASS="REPLACEABLE"
><I
>count</I
></TT
> rows to be returned.
</P
><P
> When using <TT
CLASS="LITERAL"
>LIMIT</TT
>, it is a good idea to use an
<TT
CLASS="LITERAL"
>ORDER BY</TT
> clause that constrains the result rows into a
unique order. Otherwise you will get an unpredictable subset of
the query's rows — you might be asking for the tenth through
twentieth rows, but tenth through twentieth in what ordering? You
don't know what ordering unless you specify <TT
CLASS="LITERAL"
>ORDER BY</TT
>.
</P
><P
> The query planner takes <TT
CLASS="LITERAL"
>LIMIT</TT
> into account when
generating a query plan, so you are very likely to get different
plans (yielding different row orders) depending on what you use
for <TT
CLASS="LITERAL"
>LIMIT</TT
> and <TT
CLASS="LITERAL"
>OFFSET</TT
>. Thus, using
different <TT
CLASS="LITERAL"
>LIMIT</TT
>/<TT
CLASS="LITERAL"
>OFFSET</TT
> values to select
different subsets of a query result <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>will give
inconsistent results</I
></SPAN
> unless you enforce a predictable
result ordering with <TT
CLASS="LITERAL"
>ORDER BY</TT
>. This is not a bug; it
is an inherent consequence of the fact that SQL does not promise
to deliver the results of a query in any particular order unless
<TT
CLASS="LITERAL"
>ORDER BY</TT
> is used to constrain the order.
</P
><P
> It is even possible for repeated executions of the same <TT
CLASS="LITERAL"
>LIMIT</TT
>
query to return different subsets of the rows of a table, if there
is not an <TT
CLASS="LITERAL"
>ORDER BY</TT
> to enforce selection of a deterministic
subset. Again, this is not a bug; determinism of the results is
simply not guaranteed in such a case.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="SQL-FOR-UPDATE-SHARE"
></A
><H3
><TT
CLASS="LITERAL"
>FOR UPDATE</TT
>/<TT
CLASS="LITERAL"
>FOR SHARE</TT
> Clause</H3
><P
> The <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> clause has this form:
</P><PRE
CLASS="SYNOPSIS"
>FOR UPDATE [ OF <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
> [, ...] ] [ NOWAIT ]</PRE
><P>
</P
><P
> The closely related <TT
CLASS="LITERAL"
>FOR SHARE</TT
> clause has this form:
</P><PRE
CLASS="SYNOPSIS"
>FOR SHARE [ OF <TT
CLASS="REPLACEABLE"
><I
>table_name</I
></TT
> [, ...] ] [ NOWAIT ]</PRE
><P>
</P
><P
> <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> causes the rows retrieved by the
<TT
CLASS="COMMAND"
>SELECT</TT
> statement to be locked as though for
update. This prevents them from being modified or deleted by
other transactions until the current transaction ends. That is,
other transactions that attempt <TT
CLASS="COMMAND"
>UPDATE</TT
>,
<TT
CLASS="COMMAND"
>DELETE</TT
>, or <TT
CLASS="COMMAND"
>SELECT FOR UPDATE</TT
>
of these rows will be blocked until the current transaction ends.
Also, if an <TT
CLASS="COMMAND"
>UPDATE</TT
>, <TT
CLASS="COMMAND"
>DELETE</TT
>,
or <TT
CLASS="COMMAND"
>SELECT FOR UPDATE</TT
> from another transaction
has already locked a selected row or rows, <TT
CLASS="COMMAND"
>SELECT FOR
UPDATE</TT
> will wait for the other transaction to complete,
and will then lock and return the updated row (or no row, if the
row was deleted). For further discussion see <A
HREF="mvcc.html"
>Chapter 13</A
>.
</P
><P
> To prevent the operation from waiting for other transactions to commit,
use the <TT
CLASS="LITERAL"
>NOWAIT</TT
> option. <TT
CLASS="COMMAND"
>SELECT FOR UPDATE
NOWAIT</TT
> reports an error, rather than waiting, if a selected row
cannot be locked immediately. Note that <TT
CLASS="LITERAL"
>NOWAIT</TT
> applies only
to the row-level lock(s) — the required <TT
CLASS="LITERAL"
>ROW SHARE</TT
>
table-level lock is still taken in the ordinary way (see
<A
HREF="mvcc.html"
>Chapter 13</A
>). You can use the <TT
CLASS="LITERAL"
>NOWAIT</TT
> option of
<A
HREF="sql-lock.html"
><I
>LOCK</I
></A
>
if you need to acquire the table-level lock without waiting.
</P
><P
> <TT
CLASS="LITERAL"
>FOR SHARE</TT
> behaves similarly, except that it
acquires a shared rather than exclusive lock on each retrieved
row. A shared lock blocks other transactions from performing
<TT
CLASS="COMMAND"
>UPDATE</TT
>, <TT
CLASS="COMMAND"
>DELETE</TT
>, or <TT
CLASS="COMMAND"
>SELECT
FOR UPDATE</TT
> on these rows, but it does not prevent them
from performing <TT
CLASS="COMMAND"
>SELECT FOR SHARE</TT
>.
</P
><P
> If specific tables are named in <TT
CLASS="LITERAL"
>FOR UPDATE</TT
>
or <TT
CLASS="LITERAL"
>FOR SHARE</TT
>,
then only rows coming from those tables are locked; any other
tables used in the <TT
CLASS="COMMAND"
>SELECT</TT
> are simply read as
usual. A <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
>
clause without a table list affects all tables used in the command.
If <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> or <TT
CLASS="LITERAL"
>FOR SHARE</TT
> is
applied to a view or sub-query, it affects all tables used in
the view or sub-query.
</P
><P
> Multiple <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> and <TT
CLASS="LITERAL"
>FOR SHARE</TT
>
clauses can be written if it is necessary to specify different locking
behavior for different tables. If the same table is mentioned (or
implicitly affected) by both <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> and
<TT
CLASS="LITERAL"
>FOR SHARE</TT
> clauses, then it is processed as
<TT
CLASS="LITERAL"
>FOR UPDATE</TT
>. Similarly, a table is processed
as <TT
CLASS="LITERAL"
>NOWAIT</TT
> if that is specified in any of the clauses
affecting it.
</P
><P
> <TT
CLASS="LITERAL"
>FOR UPDATE</TT
> and <TT
CLASS="LITERAL"
>FOR SHARE</TT
> cannot be
used in contexts where returned rows cannot be clearly identified with
individual table rows; for example they cannot be used with aggregation.
</P
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> Avoid locking a row and then modifying it within a later savepoint or
<SPAN
CLASS="APPLICATION"
>PL/pgSQL</SPAN
> exception block. A subsequent
rollback would cause the lock to be lost. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>BEGIN;
SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
SAVEPOINT s;
UPDATE mytable SET ... WHERE key = 1;
ROLLBACK TO s;</PRE
><P>
After the <TT
CLASS="COMMAND"
>ROLLBACK</TT
>, the row is effectively unlocked, rather
than returned to its pre-savepoint state of being locked but not modified.
This hazard occurs if a row locked in the current transaction is updated
or deleted, or if a shared lock is upgraded to exclusive: in all these
cases, the former lock state is forgotten. If the transaction is then
rolled back to a state between the original locking command and the
subsequent change, the row will appear not to be locked at all. This is
an implementation deficiency which will be addressed in a future release
of <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> It is possible for a <TT
CLASS="COMMAND"
>SELECT</TT
> command using both
<TT
CLASS="LITERAL"
>LIMIT</TT
> and <TT
CLASS="LITERAL"
>FOR UPDATE/SHARE</TT
>
clauses to return fewer rows than specified by <TT
CLASS="LITERAL"
>LIMIT</TT
>.
This is because <TT
CLASS="LITERAL"
>LIMIT</TT
> is applied first. The command
selects the specified number of rows,
but might then block trying to obtain lock on one or more of them.
Once the <TT
CLASS="LITERAL"
>SELECT</TT
> unblocks, the row might have been deleted
or updated so that it does not meet the query <TT
CLASS="LITERAL"
>WHERE</TT
> condition
anymore, in which case it will not be returned.
</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="CAUTION"
><P
></P
><TABLE
CLASS="CAUTION"
BORDER="1"
WIDTH="100%"
><TR
><TD
ALIGN="CENTER"
><B
>Caution</B
></TD
></TR
><TR
><TD
ALIGN="LEFT"
><P
> Similarly, it is possible for a <TT
CLASS="COMMAND"
>SELECT</TT
> command
using <TT
CLASS="LITERAL"
>ORDER BY</TT
> and <TT
CLASS="LITERAL"
>FOR
UPDATE/SHARE</TT
> to return rows out of order. This is
because <TT
CLASS="LITERAL"
>ORDER BY</TT
> is applied first. The command
orders the result, but might then block trying to obtain a lock
on one or more of the rows. Once the <TT
CLASS="LITERAL"
>SELECT</TT
>
unblocks, one of the ordered columns might have been modified
and be returned out of order. A workaround is to perform
<TT
CLASS="COMMAND"
>SELECT ... FOR UPDATE/SHARE</TT
> and then <TT
CLASS="COMMAND"
>SELECT
... ORDER BY</TT
>.
</P
></TD
></TR
></TABLE
></DIV
></DIV
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN60043"
></A
><H2
>Examples</H2
><P
> To join the table <TT
CLASS="LITERAL"
>films</TT
> with the table
<TT
CLASS="LITERAL"
>distributors</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT f.title, f.did, d.name, f.date_prod, f.kind
FROM distributors d, films f
WHERE f.did = d.did
title | did | name | date_prod | kind
-------------------+-----+--------------+------------+----------
The Third Man | 101 | British Lion | 1949-12-23 | Drama
The African Queen | 101 | British Lion | 1951-08-11 | Romantic
...</PRE
><P>
</P
><P
> To sum the column <TT
CLASS="LITERAL"
>len</TT
> of all films and group
the results by <TT
CLASS="LITERAL"
>kind</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT kind, sum(len) AS total FROM films GROUP BY kind;
kind | total
----------+-------
Action | 07:34
Comedy | 02:58
Drama | 14:28
Musical | 06:42
Romantic | 04:38</PRE
><P>
</P
><P
> To sum the column <TT
CLASS="LITERAL"
>len</TT
> of all films, group
the results by <TT
CLASS="LITERAL"
>kind</TT
> and show those group totals
that are less than 5 hours:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT kind, sum(len) AS total
FROM films
GROUP BY kind
HAVING sum(len) < interval '5 hours';
kind | total
----------+-------
Comedy | 02:58
Romantic | 04:38</PRE
><P>
</P
><P
> The following two examples are identical ways of sorting the individual
results according to the contents of the second column
(<TT
CLASS="LITERAL"
>name</TT
>):
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM distributors ORDER BY name;
SELECT * FROM distributors ORDER BY 2;
did | name
-----+------------------
109 | 20th Century Fox
110 | Bavaria Atelier
101 | British Lion
107 | Columbia
102 | Jean Luc Godard
113 | Luso films
104 | Mosfilm
103 | Paramount
106 | Toho
105 | United Artists
111 | Walt Disney
112 | Warner Bros.
108 | Westward</PRE
><P>
</P
><P
> The next example shows how to obtain the union of the tables
<TT
CLASS="LITERAL"
>distributors</TT
> and
<TT
CLASS="LITERAL"
>actors</TT
>, restricting the results to those that begin
with the letter W in each table. Only distinct rows are wanted, so the
key word <TT
CLASS="LITERAL"
>ALL</TT
> is omitted.
</P><PRE
CLASS="PROGRAMLISTING"
>distributors: actors:
did | name id | name
-----+-------------- ----+----------------
108 | Westward 1 | Woody Allen
111 | Walt Disney 2 | Warren Beatty
112 | Warner Bros. 3 | Walter Matthau
... ...
SELECT distributors.name
FROM distributors
WHERE distributors.name LIKE 'W%'
UNION
SELECT actors.name
FROM actors
WHERE actors.name LIKE 'W%';
name
----------------
Walt Disney
Walter Matthau
Warner Bros.
Warren Beatty
Westward
Woody Allen</PRE
><P>
</P
><P
> This example shows how to use a function in the <TT
CLASS="LITERAL"
>FROM</TT
>
clause, both with and without a column definition list:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;
SELECT * FROM distributors(111);
did | name
-----+-------------
111 | Walt Disney
CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
SELECT * FROM distributors WHERE did = $1;
$$ LANGUAGE SQL;
SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
f1 | f2
-----+-------------
111 | Walt Disney</PRE
><P>
</P
></DIV
><DIV
CLASS="REFSECT1"
><A
NAME="AEN60068"
></A
><H2
>Compatibility</H2
><P
> Of course, the <TT
CLASS="COMMAND"
>SELECT</TT
> statement is compatible
with the SQL standard. But there are some extensions and some
missing features.
</P
><DIV
CLASS="REFSECT2"
><A
NAME="AEN60072"
></A
><H3
>Omitted <TT
CLASS="LITERAL"
>FROM</TT
> Clauses</H3
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> allows one to omit the
<TT
CLASS="LITERAL"
>FROM</TT
> clause. It has a straightforward use to
compute the results of simple expressions:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT 2+2;
?column?
----------
4</PRE
><P>
Some other <ACRONYM
CLASS="ACRONYM"
>SQL</ACRONYM
> databases cannot do this except
by introducing a dummy one-row table from which to do the
<TT
CLASS="COMMAND"
>SELECT</TT
>.
</P
><P
> Note that if a <TT
CLASS="LITERAL"
>FROM</TT
> clause is not specified,
the query cannot reference any database tables. For example, the
following query is invalid:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT distributors.* WHERE distributors.name = 'Westward';</PRE
><P>
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> releases prior to
8.1 would accept queries of this form, and add an implicit entry
to the query's <TT
CLASS="LITERAL"
>FROM</TT
> clause for each table
referenced by the query. This is no longer the default behavior,
because it does not comply with the SQL standard, and is
considered by many to be error-prone. For compatibility with
applications that rely on this behavior the <A
HREF="runtime-config-compatible.html#GUC-ADD-MISSING-FROM"
>add_missing_from</A
> configuration variable can be
enabled.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN60087"
></A
><H3
>The <TT
CLASS="LITERAL"
>AS</TT
> Key Word</H3
><P
> In the SQL standard, the optional key word <TT
CLASS="LITERAL"
>AS</TT
> is just
noise and can be omitted without affecting the meaning. The
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> parser requires this key
word when renaming output columns because the type extensibility
features lead to parsing ambiguities without it.
<TT
CLASS="LITERAL"
>AS</TT
> is optional in <TT
CLASS="LITERAL"
>FROM</TT
>
items, however.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN60095"
></A
><H3
>Namespace Available to <TT
CLASS="LITERAL"
>GROUP BY</TT
> and <TT
CLASS="LITERAL"
>ORDER BY</TT
></H3
><P
> In the SQL-92 standard, an <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause can
only use result column names or numbers, while a <TT
CLASS="LITERAL"
>GROUP
BY</TT
> clause can only use expressions based on input column
names. <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extends each of
these clauses to allow the other choice as well (but it uses the
standard's interpretation if there is ambiguity).
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> also allows both clauses to
specify arbitrary expressions. Note that names appearing in an
expression will always be taken as input-column names, not as
result-column names.
</P
><P
> SQL:1999 and later use a slightly different definition which is not
entirely upward compatible with SQL-92.
In most cases, however, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>
will interpret an <TT
CLASS="LITERAL"
>ORDER BY</TT
> or <TT
CLASS="LITERAL"
>GROUP
BY</TT
> expression the same way SQL:1999 does.
</P
></DIV
><DIV
CLASS="REFSECT2"
><A
NAME="AEN60108"
></A
><H3
>Nonstandard Clauses</H3
><P
> The clauses <TT
CLASS="LITERAL"
>DISTINCT ON</TT
>,
<TT
CLASS="LITERAL"
>LIMIT</TT
>, and <TT
CLASS="LITERAL"
>OFFSET</TT
> are not
defined in the SQL standard.
</P
></DIV
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>
distrib
>
Mandriva
>
2008.1
>
x86_64
>
media
>
main-testing
>
by-pkgid
>
bab02a23fa9f3df8d66a9a3231b50245
>
files
>
812
