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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="QUERIES-TABLE-EXPRESSIONS"
>7.2. Table Expressions</A
></H1
><P
> A <I
CLASS="FIRSTTERM"
>table expression</I
> computes a table. The
table expression contains a <TT
CLASS="LITERAL"
>FROM</TT
> clause that is
optionally followed by <TT
CLASS="LITERAL"
>WHERE</TT
>, <TT
CLASS="LITERAL"
>GROUP BY</TT
>, and
<TT
CLASS="LITERAL"
>HAVING</TT
> clauses. Trivial table expressions simply refer
to a table on disk, a so-called base table, but more complex
expressions can be used to modify or combine base tables in various
ways.
</P
><P
> The optional <TT
CLASS="LITERAL"
>WHERE</TT
>, <TT
CLASS="LITERAL"
>GROUP BY</TT
>, and
<TT
CLASS="LITERAL"
>HAVING</TT
> clauses in the table expression specify a
pipeline of successive transformations performed on the table
derived in the <TT
CLASS="LITERAL"
>FROM</TT
> clause. All these transformations
produce a virtual table that provides the rows that are passed to
the select list to compute the output rows of the query.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-FROM"
>7.2.1. The <TT
CLASS="LITERAL"
>FROM</TT
> Clause</A
></H2
><P
> The <A
HREF="sql-select.html#SQL-FROM"
><I
><I
>FROM</I
> Clause</I
></A
> derives a
table from one or more other tables given in a comma-separated
table reference list.
</P><PRE
CLASS="SYNOPSIS"
>FROM <TT
CLASS="REPLACEABLE"
><I
>table_reference</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>table_reference</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ...</SPAN
>]</SPAN
>]</PRE
><P>
A table reference can be a table name (possibly schema-qualified),
or a derived table such as a subquery, a <TT
CLASS="LITERAL"
>JOIN</TT
> construct, or
complex combinations of these. If more than one table reference is
listed in the <TT
CLASS="LITERAL"
>FROM</TT
> clause, the tables are cross-joined
(that is, the Cartesian product of their rows is formed; see below).
The result of the <TT
CLASS="LITERAL"
>FROM</TT
> list is an intermediate virtual
table that can then be subject to
transformations by the <TT
CLASS="LITERAL"
>WHERE</TT
>, <TT
CLASS="LITERAL"
>GROUP BY</TT
>,
and <TT
CLASS="LITERAL"
>HAVING</TT
> clauses and is finally the result of the
overall table expression.
</P
><P
> When a table reference names a table that is the parent of a
table inheritance hierarchy, the table reference produces rows of
not only that table but all of its descendant tables, unless the
key word <TT
CLASS="LITERAL"
>ONLY</TT
> precedes the table name. However, the
reference produces only the columns that appear in the named table
— any columns added in subtables are ignored.
</P
><P
> Instead of writing <TT
CLASS="LITERAL"
>ONLY</TT
> before the table name, you can write
<TT
CLASS="LITERAL"
>*</TT
> after the table name to explicitly specify that descendant
tables are included. Writing <TT
CLASS="LITERAL"
>*</TT
> is not necessary since that
behavior is the default (unless you have changed the setting of the <A
HREF="runtime-config-compatible.html#GUC-SQL-INHERITANCE"
>sql_inheritance</A
> configuration option). However writing
<TT
CLASS="LITERAL"
>*</TT
> might be useful to emphasize that additional tables will be
searched.
</P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-JOIN"
>7.2.1.1. Joined Tables</A
></H3
><P
> A joined table is a table derived from two other (real or
derived) tables according to the rules of the particular join
type. Inner, outer, and cross-joins are available.
The general syntax of a joined table is
</P><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>join_type</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> [<SPAN
CLASS="OPTIONAL"
> <TT
CLASS="REPLACEABLE"
><I
>join_condition</I
></TT
> </SPAN
>]</PRE
><P>
Joins of all types can be chained together, or nested: either or
both <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> and
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> can be joined tables. Parentheses
can be used around <TT
CLASS="LITERAL"
>JOIN</TT
> clauses to control the join
order. In the absence of parentheses, <TT
CLASS="LITERAL"
>JOIN</TT
> clauses
nest left-to-right.
</P
><P
></P
><DIV
CLASS="VARIABLELIST"
><P
><B
>Join Types</B
></P
><DL
><DT
>Cross join
</DT
><DD
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> CROSS JOIN <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
></PRE
><P
> For every possible combination of rows from
<TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> and
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> (i.e., a Cartesian product),
the joined table will contain a
row consisting of all columns in <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>
followed by all columns in <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
>. If
the tables have N and M rows respectively, the joined
table will have N * M rows.
</P
><P
> <TT
CLASS="LITERAL"
>FROM <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> CROSS JOIN
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
></TT
> is equivalent to
<TT
CLASS="LITERAL"
>FROM <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> INNER JOIN
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> ON TRUE</TT
> (see below).
It is also equivalent to
<TT
CLASS="LITERAL"
>FROM <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>,
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
></TT
>.
</P><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> This latter equivalence does not hold exactly when more than two
tables appear, because <TT
CLASS="LITERAL"
>JOIN</TT
> binds more tightly than
comma. For example
<TT
CLASS="LITERAL"
>FROM <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> CROSS JOIN
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> INNER JOIN <TT
CLASS="REPLACEABLE"
><I
>T3</I
></TT
>
ON <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
></TT
>
is not the same as
<TT
CLASS="LITERAL"
>FROM <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>,
<TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> INNER JOIN <TT
CLASS="REPLACEABLE"
><I
>T3</I
></TT
>
ON <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
></TT
>
because the <TT
CLASS="REPLACEABLE"
><I
>condition</I
></TT
> can
reference <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> in the first case but not
the second.
</P
></BLOCKQUOTE
></DIV
><P>
</P
></DD
><DT
>Qualified joins
</DT
><DD
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> { [<SPAN
CLASS="OPTIONAL"
>INNER</SPAN
>] | { LEFT | RIGHT | FULL } [<SPAN
CLASS="OPTIONAL"
>OUTER</SPAN
>] } JOIN <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> ON <TT
CLASS="REPLACEABLE"
><I
>boolean_expression</I
></TT
>
<TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> { [<SPAN
CLASS="OPTIONAL"
>INNER</SPAN
>] | { LEFT | RIGHT | FULL } [<SPAN
CLASS="OPTIONAL"
>OUTER</SPAN
>] } JOIN <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> USING ( <TT
CLASS="REPLACEABLE"
><I
>join column list</I
></TT
> )
<TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> NATURAL { [<SPAN
CLASS="OPTIONAL"
>INNER</SPAN
>] | { LEFT | RIGHT | FULL } [<SPAN
CLASS="OPTIONAL"
>OUTER</SPAN
>] } JOIN <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
></PRE
><P
> The words <TT
CLASS="LITERAL"
>INNER</TT
> and
<TT
CLASS="LITERAL"
>OUTER</TT
> are optional in all forms.
<TT
CLASS="LITERAL"
>INNER</TT
> is the default;
<TT
CLASS="LITERAL"
>LEFT</TT
>, <TT
CLASS="LITERAL"
>RIGHT</TT
>, and
<TT
CLASS="LITERAL"
>FULL</TT
> imply an outer join.
</P
><P
> The <I
CLASS="FIRSTTERM"
>join condition</I
> is specified in the
<TT
CLASS="LITERAL"
>ON</TT
> or <TT
CLASS="LITERAL"
>USING</TT
> clause, or implicitly by
the word <TT
CLASS="LITERAL"
>NATURAL</TT
>. The join condition determines
which rows from the two source tables are considered to
<SPAN
CLASS="QUOTE"
>"match"</SPAN
>, as explained in detail below.
</P
><P
> The possible types of qualified join are:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
><TT
CLASS="LITERAL"
>INNER JOIN</TT
></DT
><DD
><P
> For each row R1 of T1, the joined table has a row for each
row in T2 that satisfies the join condition with R1.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>LEFT OUTER JOIN</TT
>
</DT
><DD
><P
> First, an inner join is performed. Then, for each row in
T1 that does not satisfy the join condition with any row in
T2, a joined row is added with null values in columns of
T2. Thus, the joined table always has at least
one row for each row in T1.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>RIGHT OUTER JOIN</TT
>
</DT
><DD
><P
> First, an inner join is performed. Then, for each row in
T2 that does not satisfy the join condition with any row in
T1, a joined row is added with null values in columns of
T1. This is the converse of a left join: the result table
will always have a row for each row in T2.
</P
></DD
><DT
><TT
CLASS="LITERAL"
>FULL OUTER JOIN</TT
></DT
><DD
><P
> First, an inner join is performed. Then, for each row in
T1 that does not satisfy the join condition with any row in
T2, a joined row is added with null values in columns of
T2. Also, for each row of T2 that does not satisfy the
join condition with any row in T1, a joined row with null
values in the columns of T1 is added.
</P
></DD
></DL
></DIV
><P>
</P
><P
> The <TT
CLASS="LITERAL"
>ON</TT
> clause is the most general kind of join
condition: it takes a Boolean value expression of the same
kind as is used in a <TT
CLASS="LITERAL"
>WHERE</TT
> clause. A pair of rows
from <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> and <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> match if the
<TT
CLASS="LITERAL"
>ON</TT
> expression evaluates to true.
</P
><P
> The <TT
CLASS="LITERAL"
>USING</TT
> clause is a shorthand that allows you to take
advantage of the specific situation where both sides of the join use
the same name for the joining column(s). It takes a
comma-separated list of the shared column names
and forms a join condition that includes an equality comparison
for each one. For example, joining <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>
and <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> with <TT
CLASS="LITERAL"
>USING (a, b)</TT
> produces
the join condition <TT
CLASS="LITERAL"
>ON <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>.a
= <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
>.a AND <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>.b
= <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
>.b</TT
>.
</P
><P
> Furthermore, the output of <TT
CLASS="LITERAL"
>JOIN USING</TT
> suppresses
redundant columns: there is no need to print both of the matched
columns, since they must have equal values. While <TT
CLASS="LITERAL"
>JOIN
ON</TT
> produces all columns from <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> followed by all
columns from <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
>, <TT
CLASS="LITERAL"
>JOIN USING</TT
> produces one
output column for each of the listed column pairs (in the listed
order), followed by any remaining columns from <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
>,
followed by any remaining columns from <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
>.
</P
><P
>
Finally, <TT
CLASS="LITERAL"
>NATURAL</TT
> is a shorthand form of
<TT
CLASS="LITERAL"
>USING</TT
>: it forms a <TT
CLASS="LITERAL"
>USING</TT
> list
consisting of all column names that appear in both
input tables. As with <TT
CLASS="LITERAL"
>USING</TT
>, these columns appear
only once in the output table. If there are no common
column names, <TT
CLASS="LITERAL"
>NATURAL JOIN</TT
> behaves like
<TT
CLASS="LITERAL"
>JOIN ... ON TRUE</TT
>, producing a cross-product join.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> <TT
CLASS="LITERAL"
>USING</TT
> is reasonably safe from column changes
in the joined relations since only the listed columns
are combined. <TT
CLASS="LITERAL"
>NATURAL</TT
> is considerably more risky since
any schema changes to either relation that cause a new matching
column name to be present will cause the join to combine that new
column as well.
</P
></BLOCKQUOTE
></DIV
></DD
></DL
></DIV
><P
> To put this together, assume we have tables <TT
CLASS="LITERAL"
>t1</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
> num | name
-----+------
1 | a
2 | b
3 | c</PRE
><P>
and <TT
CLASS="LITERAL"
>t2</TT
>:
</P><PRE
CLASS="PROGRAMLISTING"
> num | value
-----+-------
1 | xxx
3 | yyy
5 | zzz</PRE
><P>
then we get the following results for the various joins:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 CROSS JOIN t2;</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
1 | a | 3 | yyy
1 | a | 5 | zzz
2 | b | 1 | xxx
2 | b | 3 | yyy
2 | b | 5 | zzz
3 | c | 1 | xxx
3 | c | 3 | yyy
3 | c | 5 | zzz
(9 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 INNER JOIN t2 ON t1.num = t2.num;</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
3 | c | 3 | yyy
(2 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 INNER JOIN t2 USING (num);</KBD
>
num | name | value
-----+------+-------
1 | a | xxx
3 | c | yyy
(2 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 NATURAL INNER JOIN t2;</KBD
>
num | name | value
-----+------+-------
1 | a | xxx
3 | c | yyy
(2 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num;</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
2 | b | |
3 | c | 3 | yyy
(3 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 LEFT JOIN t2 USING (num);</KBD
>
num | name | value
-----+------+-------
1 | a | xxx
2 | b |
3 | c | yyy
(3 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 RIGHT JOIN t2 ON t1.num = t2.num;</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
3 | c | 3 | yyy
| | 5 | zzz
(3 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 FULL JOIN t2 ON t1.num = t2.num;</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
2 | b | |
3 | c | 3 | yyy
| | 5 | zzz
(4 rows)</PRE
><P>
</P
><P
> The join condition specified with <TT
CLASS="LITERAL"
>ON</TT
> can also contain
conditions that do not relate directly to the join. This can
prove useful for some queries but needs to be thought out
carefully. For example:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.value = 'xxx';</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
2 | b | |
3 | c | |
(3 rows)</PRE
><P>
Notice that placing the restriction in the <TT
CLASS="LITERAL"
>WHERE</TT
> clause
produces a different result:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num WHERE t2.value = 'xxx';</KBD
>
num | name | num | value
-----+------+-----+-------
1 | a | 1 | xxx
(1 row)</PRE
><P>
This is because a restriction placed in the <TT
CLASS="LITERAL"
>ON</TT
>
clause is processed <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>before</I
></SPAN
> the join, while
a restriction placed in the <TT
CLASS="LITERAL"
>WHERE</TT
> clause is processed
<SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>after</I
></SPAN
> the join.
That does not matter with inner joins, but it matters a lot with outer
joins.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-TABLE-ALIASES"
>7.2.1.2. Table and Column Aliases</A
></H3
><P
> A temporary name can be given to tables and complex table
references to be used for references to the derived table in
the rest of the query. This is called a <I
CLASS="FIRSTTERM"
>table
alias</I
>.
</P
><P
> To create a table alias, write
</P><PRE
CLASS="SYNOPSIS"
>FROM <TT
CLASS="REPLACEABLE"
><I
>table_reference</I
></TT
> AS <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
></PRE
><P>
or
</P><PRE
CLASS="SYNOPSIS"
>FROM <TT
CLASS="REPLACEABLE"
><I
>table_reference</I
></TT
> <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
></PRE
><P>
The <TT
CLASS="LITERAL"
>AS</TT
> key word is optional noise.
<TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
> can be any identifier.
</P
><P
> A typical application of table aliases is to assign short
identifiers to long table names to keep the join clauses
readable. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM some_very_long_table_name s JOIN another_fairly_long_name a ON s.id = a.num;</PRE
><P>
</P
><P
> The alias becomes the new name of the table reference so far as the
current query is concerned — it is not allowed to refer to the
table by the original name elsewhere in the query. Thus, this is not
valid:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM my_table AS m WHERE my_table.a > 5; -- wrong</PRE
><P>
</P
><P
> Table aliases are mainly for notational convenience, but it is
necessary to use them when joining a table to itself, e.g.:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM people AS mother JOIN people AS child ON mother.id = child.mother_id;</PRE
><P>
Additionally, an alias is required if the table reference is a
subquery (see <A
HREF="queries-table-expressions.html#QUERIES-SUBQUERIES"
>Section 7.2.1.3</A
>).
</P
><P
> Parentheses are used to resolve ambiguities. In the following example,
the first statement assigns the alias <TT
CLASS="LITERAL"
>b</TT
> to the second
instance of <TT
CLASS="LITERAL"
>my_table</TT
>, but the second statement assigns the
alias to the result of the join:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM my_table AS a CROSS JOIN my_table AS b ...
SELECT * FROM (my_table AS a CROSS JOIN my_table) AS b ...</PRE
><P>
</P
><P
> Another form of table aliasing gives temporary names to the columns of
the table, as well as the table itself:
</P><PRE
CLASS="SYNOPSIS"
>FROM <TT
CLASS="REPLACEABLE"
><I
>table_reference</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>AS</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
> ( <TT
CLASS="REPLACEABLE"
><I
>column1</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>column2</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ...</SPAN
>]</SPAN
>] )</PRE
><P>
If fewer column aliases are specified than the actual table has
columns, the remaining columns are not renamed. This syntax is
especially useful for self-joins or subqueries.
</P
><P
> When an alias is applied to the output of a <TT
CLASS="LITERAL"
>JOIN</TT
>
clause, the alias hides the original
name(s) within the <TT
CLASS="LITERAL"
>JOIN</TT
>. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a.* FROM my_table AS a JOIN your_table AS b ON ...</PRE
><P>
is valid SQL, but:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT a.* FROM (my_table AS a JOIN your_table AS b ON ...) AS c</PRE
><P>
is not valid; the table alias <TT
CLASS="LITERAL"
>a</TT
> is not visible
outside the alias <TT
CLASS="LITERAL"
>c</TT
>.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-SUBQUERIES"
>7.2.1.3. Subqueries</A
></H3
><P
> Subqueries specifying a derived table must be enclosed in
parentheses and <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>must</I
></SPAN
> be assigned a table
alias name (as in <A
HREF="queries-table-expressions.html#QUERIES-TABLE-ALIASES"
>Section 7.2.1.2</A
>). For
example:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM (SELECT * FROM table1) AS alias_name</PRE
><P>
</P
><P
> This example is equivalent to <TT
CLASS="LITERAL"
>FROM table1 AS
alias_name</TT
>. More interesting cases, which cannot be
reduced to a plain join, arise when the subquery involves
grouping or aggregation.
</P
><P
> A subquery can also be a <TT
CLASS="COMMAND"
>VALUES</TT
> list:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM (VALUES ('anne', 'smith'), ('bob', 'jones'), ('joe', 'blow'))
AS names(first, last)</PRE
><P>
Again, a table alias is required. Assigning alias names to the columns
of the <TT
CLASS="COMMAND"
>VALUES</TT
> list is optional, but is good practice.
For more information see <A
HREF="queries-values.html"
>Section 7.7</A
>.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-TABLEFUNCTIONS"
>7.2.1.4. Table Functions</A
></H3
><P
> Table functions are functions that produce a set of rows, made up
of either base data types (scalar types) or composite data types
(table rows). They are used like a table, view, or subquery in
the <TT
CLASS="LITERAL"
>FROM</TT
> clause of a query. Columns returned by table
functions can be included in <TT
CLASS="LITERAL"
>SELECT</TT
>,
<TT
CLASS="LITERAL"
>JOIN</TT
>, or <TT
CLASS="LITERAL"
>WHERE</TT
> clauses in the same manner
as columns of a table, view, or subquery.
</P
><P
> Table functions may also be combined using the <TT
CLASS="LITERAL"
>ROWS FROM</TT
>
syntax, with the results returned in parallel columns; the number of
result rows in this case is that of the largest function result, with
smaller results padded with null values to match.
</P
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>function_call</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>WITH ORDINALITY</SPAN
>] [<SPAN
CLASS="OPTIONAL"
>[<SPAN
CLASS="OPTIONAL"
>AS</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>table_alias</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>(<TT
CLASS="REPLACEABLE"
><I
>column_alias</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>])</SPAN
>]</SPAN
>]
ROWS FROM( <TT
CLASS="REPLACEABLE"
><I
>function_call</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>] ) [<SPAN
CLASS="OPTIONAL"
>WITH ORDINALITY</SPAN
>] [<SPAN
CLASS="OPTIONAL"
>[<SPAN
CLASS="OPTIONAL"
>AS</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>table_alias</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>(<TT
CLASS="REPLACEABLE"
><I
>column_alias</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>])</SPAN
>]</SPAN
>]</PRE
><P
> If the <TT
CLASS="LITERAL"
>WITH ORDINALITY</TT
> clause is specified, an
additional column of type <TT
CLASS="TYPE"
>bigint</TT
> will be added to the
function result columns. This column numbers the rows of the function
result set, starting from 1. (This is a generalization of the
SQL-standard syntax for <TT
CLASS="LITERAL"
>UNNEST ... WITH ORDINALITY</TT
>.)
By default, the ordinal column is called <TT
CLASS="LITERAL"
>ordinality</TT
>, but
a different column name can be assigned to it using
an <TT
CLASS="LITERAL"
>AS</TT
> clause.
</P
><P
> The special table function <TT
CLASS="LITERAL"
>UNNEST</TT
> may be called with
any number of array parameters, and it returns a corresponding number of
columns, as if <TT
CLASS="LITERAL"
>UNNEST</TT
>
(<A
HREF="functions-array.html"
>Section 9.18</A
>) had been called on each parameter
separately and combined using the <TT
CLASS="LITERAL"
>ROWS FROM</TT
> construct.
</P
><PRE
CLASS="SYNOPSIS"
>UNNEST( <TT
CLASS="REPLACEABLE"
><I
>array_expression</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>] ) [<SPAN
CLASS="OPTIONAL"
>WITH ORDINALITY</SPAN
>] [<SPAN
CLASS="OPTIONAL"
>[<SPAN
CLASS="OPTIONAL"
>AS</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>table_alias</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>(<TT
CLASS="REPLACEABLE"
><I
>column_alias</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>])</SPAN
>]</SPAN
>]</PRE
><P
> If no <TT
CLASS="REPLACEABLE"
><I
>table_alias</I
></TT
> is specified, the function
name is used as the table name; in the case of a <TT
CLASS="LITERAL"
>ROWS FROM()</TT
>
construct, the first function's name is used.
</P
><P
> If column aliases are not supplied, then for a function returning a base
data type, the column name is also the same as the function name. For a
function returning a composite type, the result columns get the names
of the individual attributes of the type.
</P
><P
> Some examples:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE foo (fooid int, foosubid int, fooname text);
CREATE FUNCTION getfoo(int) RETURNS SETOF foo AS $$
SELECT * FROM foo WHERE fooid = $1;
$$ LANGUAGE SQL;
SELECT * FROM getfoo(1) AS t1;
SELECT * FROM foo
WHERE foosubid IN (
SELECT foosubid
FROM getfoo(foo.fooid) z
WHERE z.fooid = foo.fooid
);
CREATE VIEW vw_getfoo AS SELECT * FROM getfoo(1);
SELECT * FROM vw_getfoo;</PRE
><P>
</P
><P
> In some cases it is useful to define table functions that can
return different column sets depending on how they are invoked.
To support this, the table function can be declared as returning
the pseudotype <TT
CLASS="TYPE"
>record</TT
>. When such a function is used in
a query, the expected row structure must be specified in the
query itself, so that the system can know how to parse and plan
the query. This syntax looks like:
</P
><PRE
CLASS="SYNOPSIS"
><TT
CLASS="REPLACEABLE"
><I
>function_call</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>AS</SPAN
>] <TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
> (<TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>])
<TT
CLASS="REPLACEABLE"
><I
>function_call</I
></TT
> AS [<SPAN
CLASS="OPTIONAL"
><TT
CLASS="REPLACEABLE"
><I
>alias</I
></TT
></SPAN
>] (<TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>])
ROWS FROM( ... <TT
CLASS="REPLACEABLE"
><I
>function_call</I
></TT
> AS (<TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>]) [<SPAN
CLASS="OPTIONAL"
>, ... </SPAN
>] )</PRE
><P
> When not using the <TT
CLASS="LITERAL"
>ROWS FROM()</TT
> syntax,
the <TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> list replaces the column
alias list that could otherwise be attached to the <TT
CLASS="LITERAL"
>FROM</TT
>
item; the names in the column definitions serve as column aliases.
When using the <TT
CLASS="LITERAL"
>ROWS FROM()</TT
> syntax,
a <TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> list can be attached to
each member function separately; or if there is only one member function
and no <TT
CLASS="LITERAL"
>WITH ORDINALITY</TT
> clause,
a <TT
CLASS="REPLACEABLE"
><I
>column_definition</I
></TT
> list can be written in
place of a column alias list following <TT
CLASS="LITERAL"
>ROWS FROM()</TT
>.
</P
><P
> Consider this example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT *
FROM dblink('dbname=mydb', 'SELECT proname, prosrc FROM pg_proc')
AS t1(proname name, prosrc text)
WHERE proname LIKE 'bytea%';</PRE
><P>
The <A
HREF="contrib-dblink-function.html"
>dblink</A
> function
(part of the <A
HREF="dblink.html"
>dblink</A
> module) executes
a remote query. It is declared to return
<TT
CLASS="TYPE"
>record</TT
> since it might be used for any kind of query.
The actual column set must be specified in the calling query so
that the parser knows, for example, what <TT
CLASS="LITERAL"
>*</TT
> should
expand to.
</P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-LATERAL"
>7.2.1.5. <TT
CLASS="LITERAL"
>LATERAL</TT
> Subqueries</A
></H3
><P
> Subqueries appearing in <TT
CLASS="LITERAL"
>FROM</TT
> can be
preceded by the key word <TT
CLASS="LITERAL"
>LATERAL</TT
>. This allows them to
reference columns provided by preceding <TT
CLASS="LITERAL"
>FROM</TT
> items.
(Without <TT
CLASS="LITERAL"
>LATERAL</TT
>, each subquery is
evaluated independently and so cannot cross-reference any other
<TT
CLASS="LITERAL"
>FROM</TT
> item.)
</P
><P
> Table functions appearing in <TT
CLASS="LITERAL"
>FROM</TT
> can also be
preceded by the key word <TT
CLASS="LITERAL"
>LATERAL</TT
>, but for functions the
key word is optional; the function's arguments can contain references
to columns provided by preceding <TT
CLASS="LITERAL"
>FROM</TT
> items in any case.
</P
><P
> A <TT
CLASS="LITERAL"
>LATERAL</TT
> item can appear at top level in the
<TT
CLASS="LITERAL"
>FROM</TT
> list, or within a <TT
CLASS="LITERAL"
>JOIN</TT
> tree. In the latter
case it can also refer to any items that are on the left-hand side of a
<TT
CLASS="LITERAL"
>JOIN</TT
> that it is on the right-hand side of.
</P
><P
> When a <TT
CLASS="LITERAL"
>FROM</TT
> item contains <TT
CLASS="LITERAL"
>LATERAL</TT
>
cross-references, evaluation proceeds as follows: for each row of the
<TT
CLASS="LITERAL"
>FROM</TT
> item providing the cross-referenced column(s), or
set of rows of multiple <TT
CLASS="LITERAL"
>FROM</TT
> items providing the
columns, the <TT
CLASS="LITERAL"
>LATERAL</TT
> item is evaluated using that
row or row set's values of the columns. The resulting row(s) are
joined as usual with the rows they were computed from. This is
repeated for each row or set of rows from the column source table(s).
</P
><P
> A trivial example of <TT
CLASS="LITERAL"
>LATERAL</TT
> is
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM foo, LATERAL (SELECT * FROM bar WHERE bar.id = foo.bar_id) ss;</PRE
><P>
This is not especially useful since it has exactly the same result as
the more conventional
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM foo, bar WHERE bar.id = foo.bar_id;</PRE
><P>
<TT
CLASS="LITERAL"
>LATERAL</TT
> is primarily useful when the cross-referenced
column is necessary for computing the row(s) to be joined. A common
application is providing an argument value for a set-returning function.
For example, supposing that <CODE
CLASS="FUNCTION"
>vertices(polygon)</CODE
> returns the
set of vertices of a polygon, we could identify close-together vertices
of polygons stored in a table with:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT p1.id, p2.id, v1, v2
FROM polygons p1, polygons p2,
LATERAL vertices(p1.poly) v1,
LATERAL vertices(p2.poly) v2
WHERE (v1 <-> v2) < 10 AND p1.id != p2.id;</PRE
><P>
This query could also be written
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT p1.id, p2.id, v1, v2
FROM polygons p1 CROSS JOIN LATERAL vertices(p1.poly) v1,
polygons p2 CROSS JOIN LATERAL vertices(p2.poly) v2
WHERE (v1 <-> v2) < 10 AND p1.id != p2.id;</PRE
><P>
or in several other equivalent formulations. (As already mentioned,
the <TT
CLASS="LITERAL"
>LATERAL</TT
> key word is unnecessary in this example, but
we use it for clarity.)
</P
><P
> It is often particularly handy to <TT
CLASS="LITERAL"
>LEFT JOIN</TT
> to a
<TT
CLASS="LITERAL"
>LATERAL</TT
> subquery, so that source rows will appear in
the result even if the <TT
CLASS="LITERAL"
>LATERAL</TT
> subquery produces no
rows for them. For example, if <CODE
CLASS="FUNCTION"
>get_product_names()</CODE
> returns
the names of products made by a manufacturer, but some manufacturers in
our table currently produce no products, we could find out which ones
those are like this:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT m.name
FROM manufacturers m LEFT JOIN LATERAL get_product_names(m.id) pname ON true
WHERE pname IS NULL;</PRE
><P>
</P
></DIV
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-WHERE"
>7.2.2. The <TT
CLASS="LITERAL"
>WHERE</TT
> Clause</A
></H2
><P
> The syntax of the <A
HREF="sql-select.html#SQL-WHERE"
><I
><I
>WHERE</I
> Clause</I
></A
> is
</P><PRE
CLASS="SYNOPSIS"
>WHERE <TT
CLASS="REPLACEABLE"
><I
>search_condition</I
></TT
></PRE
><P>
where <TT
CLASS="REPLACEABLE"
><I
>search_condition</I
></TT
> is any value
expression (see <A
HREF="sql-expressions.html"
>Section 4.2</A
>) that
returns a value of type <TT
CLASS="TYPE"
>boolean</TT
>.
</P
><P
> After the processing of the <TT
CLASS="LITERAL"
>FROM</TT
> clause is done, each
row of the derived virtual table is checked against the search
condition. If the result of the condition is true, the row is
kept in the output table, otherwise (i.e., if the result is
false or null) it is discarded. The search condition typically
references at least one column of the table generated in the
<TT
CLASS="LITERAL"
>FROM</TT
> clause; this is not required, but otherwise the
<TT
CLASS="LITERAL"
>WHERE</TT
> clause will be fairly useless.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> The join condition of an inner join can be written either in
the <TT
CLASS="LITERAL"
>WHERE</TT
> clause or in the <TT
CLASS="LITERAL"
>JOIN</TT
> clause.
For example, these table expressions are equivalent:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM a, b WHERE a.id = b.id AND b.val > 5</PRE
><P>
and:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM a INNER JOIN b ON (a.id = b.id) WHERE b.val > 5</PRE
><P>
or perhaps even:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM a NATURAL JOIN b WHERE b.val > 5</PRE
><P>
Which one of these you use is mainly a matter of style. The
<TT
CLASS="LITERAL"
>JOIN</TT
> syntax in the <TT
CLASS="LITERAL"
>FROM</TT
> clause is
probably not as portable to other SQL database management systems,
even though it is in the SQL standard. For
outer joins there is no choice: they must be done in
the <TT
CLASS="LITERAL"
>FROM</TT
> clause. The <TT
CLASS="LITERAL"
>ON</TT
> or <TT
CLASS="LITERAL"
>USING</TT
>
clause of an outer join is <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> equivalent to a
<TT
CLASS="LITERAL"
>WHERE</TT
> condition, because it results in the addition
of rows (for unmatched input rows) as well as the removal of rows
in the final result.
</P
></BLOCKQUOTE
></DIV
><P
> Here are some examples of <TT
CLASS="LITERAL"
>WHERE</TT
> clauses:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT ... FROM fdt WHERE c1 > 5
SELECT ... FROM fdt WHERE c1 IN (1, 2, 3)
SELECT ... FROM fdt WHERE c1 IN (SELECT c1 FROM t2)
SELECT ... FROM fdt WHERE c1 IN (SELECT c3 FROM t2 WHERE c2 = fdt.c1 + 10)
SELECT ... FROM fdt WHERE c1 BETWEEN (SELECT c3 FROM t2 WHERE c2 = fdt.c1 + 10) AND 100
SELECT ... FROM fdt WHERE EXISTS (SELECT c1 FROM t2 WHERE c2 > fdt.c1)</PRE
><P>
<TT
CLASS="LITERAL"
>fdt</TT
> is the table derived in the
<TT
CLASS="LITERAL"
>FROM</TT
> clause. Rows that do not meet the search
condition of the <TT
CLASS="LITERAL"
>WHERE</TT
> clause are eliminated from
<TT
CLASS="LITERAL"
>fdt</TT
>. Notice the use of scalar subqueries as
value expressions. Just like any other query, the subqueries can
employ complex table expressions. Notice also how
<TT
CLASS="LITERAL"
>fdt</TT
> is referenced in the subqueries.
Qualifying <TT
CLASS="LITERAL"
>c1</TT
> as <TT
CLASS="LITERAL"
>fdt.c1</TT
> is only necessary
if <TT
CLASS="LITERAL"
>c1</TT
> is also the name of a column in the derived
input table of the subquery. But qualifying the column name adds
clarity even when it is not needed. This example shows how the column
naming scope of an outer query extends into its inner queries.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-GROUP"
>7.2.3. The <TT
CLASS="LITERAL"
>GROUP BY</TT
> and <TT
CLASS="LITERAL"
>HAVING</TT
> Clauses</A
></H2
><P
> After passing the <TT
CLASS="LITERAL"
>WHERE</TT
> filter, the derived input
table might be subject to grouping, using the <TT
CLASS="LITERAL"
>GROUP BY</TT
>
clause, and elimination of group rows using the <TT
CLASS="LITERAL"
>HAVING</TT
>
clause.
</P
><PRE
CLASS="SYNOPSIS"
>SELECT <TT
CLASS="REPLACEABLE"
><I
>select_list</I
></TT
>
FROM ...
[<SPAN
CLASS="OPTIONAL"
>WHERE ...</SPAN
>]
GROUP BY <TT
CLASS="REPLACEABLE"
><I
>grouping_column_reference</I
></TT
> [<SPAN
CLASS="OPTIONAL"
>, <TT
CLASS="REPLACEABLE"
><I
>grouping_column_reference</I
></TT
></SPAN
>]...</PRE
><P
> The <A
HREF="sql-select.html#SQL-GROUPBY"
><I
><I
>GROUP BY</I
> Clause</I
></A
> is
used to group together those rows in a table that have the same
values in all the columns listed. The order in which the columns
are listed does not matter. The effect is to combine each set
of rows having common values into one group row that
represents all rows in the group. This is done to
eliminate redundancy in the output and/or compute aggregates that
apply to these groups. For instance:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM test1;</KBD
>
x | y
---+---
a | 3
c | 2
b | 5
a | 1
(4 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT x FROM test1 GROUP BY x;</KBD
>
x
---
a
b
c
(3 rows)</PRE
><P>
</P
><P
> In the second query, we could not have written <TT
CLASS="LITERAL"
>SELECT *
FROM test1 GROUP BY x</TT
>, because there is no single value
for the column <TT
CLASS="LITERAL"
>y</TT
> that could be associated with each
group. The grouped-by columns can be referenced in the select list since
they have a single value in each group.
</P
><P
> In general, if a table is grouped, columns that are not
listed in <TT
CLASS="LITERAL"
>GROUP BY</TT
> cannot be referenced except in aggregate
expressions. An example with aggregate expressions is:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT x, sum(y) FROM test1 GROUP BY x;</KBD
>
x | sum
---+-----
a | 4
b | 5
c | 2
(3 rows)</PRE
><P>
Here <TT
CLASS="LITERAL"
>sum</TT
> is an aggregate function that
computes a single value over the entire group. More information
about the available aggregate functions can be found in <A
HREF="functions-aggregate.html"
>Section 9.20</A
>.
</P
><DIV
CLASS="TIP"
><BLOCKQUOTE
CLASS="TIP"
><P
><B
>Tip: </B
> Grouping without aggregate expressions effectively calculates the
set of distinct values in a column. This can also be achieved
using the <TT
CLASS="LITERAL"
>DISTINCT</TT
> clause (see <A
HREF="queries-select-lists.html#QUERIES-DISTINCT"
>Section 7.3.3</A
>).
</P
></BLOCKQUOTE
></DIV
><P
> Here is another example: it calculates the total sales for each
product (rather than the total sales of all products):
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT product_id, p.name, (sum(s.units) * p.price) AS sales
FROM products p LEFT JOIN sales s USING (product_id)
GROUP BY product_id, p.name, p.price;</PRE
><P>
In this example, the columns <TT
CLASS="LITERAL"
>product_id</TT
>,
<TT
CLASS="LITERAL"
>p.name</TT
>, and <TT
CLASS="LITERAL"
>p.price</TT
> must be
in the <TT
CLASS="LITERAL"
>GROUP BY</TT
> clause since they are referenced in
the query select list (but see below). The column
<TT
CLASS="LITERAL"
>s.units</TT
> does not have to be in the <TT
CLASS="LITERAL"
>GROUP
BY</TT
> list since it is only used in an aggregate expression
(<TT
CLASS="LITERAL"
>sum(...)</TT
>), which represents the sales
of a product. For each product, the query returns a summary row about
all sales of the product.
</P
><P
> If the products table is set up so that, say,
<TT
CLASS="LITERAL"
>product_id</TT
> is the primary key, then it would be
enough to group by <TT
CLASS="LITERAL"
>product_id</TT
> in the above example,
since name and price would be <I
CLASS="FIRSTTERM"
>functionally
dependent</I
> on the product ID, and so there would be no
ambiguity about which name and price value to return for each product
ID group.
</P
><P
> In strict SQL, <TT
CLASS="LITERAL"
>GROUP BY</TT
> can only group by columns of
the source table but <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> extends
this to also allow <TT
CLASS="LITERAL"
>GROUP BY</TT
> to group by columns in the
select list. Grouping by value expressions instead of simple
column names is also allowed.
</P
><P
> If a table has been grouped using <TT
CLASS="LITERAL"
>GROUP BY</TT
>,
but only certain groups are of interest, the
<TT
CLASS="LITERAL"
>HAVING</TT
> clause can be used, much like a
<TT
CLASS="LITERAL"
>WHERE</TT
> clause, to eliminate groups from the result.
The syntax is:
</P><PRE
CLASS="SYNOPSIS"
>SELECT <TT
CLASS="REPLACEABLE"
><I
>select_list</I
></TT
> FROM ... [<SPAN
CLASS="OPTIONAL"
>WHERE ...</SPAN
>] GROUP BY ... HAVING <TT
CLASS="REPLACEABLE"
><I
>boolean_expression</I
></TT
></PRE
><P>
Expressions in the <TT
CLASS="LITERAL"
>HAVING</TT
> clause can refer both to
grouped expressions and to ungrouped expressions (which necessarily
involve an aggregate function).
</P
><P
> Example:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT x, sum(y) FROM test1 GROUP BY x HAVING sum(y) > 3;</KBD
>
x | sum
---+-----
a | 4
b | 5
(2 rows)
<SAMP
CLASS="PROMPT"
>=></SAMP
> <KBD
CLASS="USERINPUT"
>SELECT x, sum(y) FROM test1 GROUP BY x HAVING x < 'c';</KBD
>
x | sum
---+-----
a | 4
b | 5
(2 rows)</PRE
><P>
</P
><P
> Again, a more realistic example:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT product_id, p.name, (sum(s.units) * (p.price - p.cost)) AS profit
FROM products p LEFT JOIN sales s USING (product_id)
WHERE s.date > CURRENT_DATE - INTERVAL '4 weeks'
GROUP BY product_id, p.name, p.price, p.cost
HAVING sum(p.price * s.units) > 5000;</PRE
><P>
In the example above, the <TT
CLASS="LITERAL"
>WHERE</TT
> clause is selecting
rows by a column that is not grouped (the expression is only true for
sales during the last four weeks), while the <TT
CLASS="LITERAL"
>HAVING</TT
>
clause restricts the output to groups with total gross sales over
5000. Note that the aggregate expressions do not necessarily need
to be the same in all parts of the query.
</P
><P
> If a query contains aggregate function calls, but no <TT
CLASS="LITERAL"
>GROUP BY</TT
>
clause, grouping still occurs: the result is a single group row (or
perhaps no rows at all, if the single row is then eliminated by
<TT
CLASS="LITERAL"
>HAVING</TT
>).
The same is true if it contains a <TT
CLASS="LITERAL"
>HAVING</TT
> clause, even
without any aggregate function calls or <TT
CLASS="LITERAL"
>GROUP BY</TT
> clause.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-WINDOW"
>7.2.4. Window Function Processing</A
></H2
><P
> If the query contains any window functions (see
<A
HREF="tutorial-window.html"
>Section 3.5</A
>,
<A
HREF="functions-window.html"
>Section 9.21</A
> and
<A
HREF="sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS"
>Section 4.2.8</A
>), these functions are evaluated
after any grouping, aggregation, and <TT
CLASS="LITERAL"
>HAVING</TT
> filtering is
performed. That is, if the query uses any aggregates, <TT
CLASS="LITERAL"
>GROUP
BY</TT
>, or <TT
CLASS="LITERAL"
>HAVING</TT
>, then the rows seen by the window functions
are the group rows instead of the original table rows from
<TT
CLASS="LITERAL"
>FROM</TT
>/<TT
CLASS="LITERAL"
>WHERE</TT
>.
</P
><P
> When multiple window functions are used, all the window functions having
syntactically equivalent <TT
CLASS="LITERAL"
>PARTITION BY</TT
> and <TT
CLASS="LITERAL"
>ORDER BY</TT
>
clauses in their window definitions are guaranteed to be evaluated in a
single pass over the data. Therefore they will see the same sort ordering,
even if the <TT
CLASS="LITERAL"
>ORDER BY</TT
> does not uniquely determine an ordering.
However, no guarantees are made about the evaluation of functions having
different <TT
CLASS="LITERAL"
>PARTITION BY</TT
> or <TT
CLASS="LITERAL"
>ORDER BY</TT
> specifications.
(In such cases a sort step is typically required between the passes of
window function evaluations, and the sort is not guaranteed to preserve
ordering of rows that its <TT
CLASS="LITERAL"
>ORDER BY</TT
> sees as equivalent.)
</P
><P
> Currently, window functions always require presorted data, and so the
query output will be ordered according to one or another of the window
functions' <TT
CLASS="LITERAL"
>PARTITION BY</TT
>/<TT
CLASS="LITERAL"
>ORDER BY</TT
> clauses.
It is not recommended to rely on this, however. Use an explicit
top-level <TT
CLASS="LITERAL"
>ORDER BY</TT
> clause if you want to be sure the
results are sorted in a particular way.
</P
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