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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="QUERIES-TABLE-EXPRESSIONS"
>7.2. Table Expressions</A
></H1
><A
NAME="AEN3018"
></A
><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 table join, or complex
    combinations of these.  If more than one table reference is listed
    in the <TT
CLASS="LITERAL"
>FROM</TT
> clause they are cross-joined (see below)
    to form the 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
><A
NAME="AEN3045"
></A
><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
    &mdash; any columns added in subtables are ignored.
   </P
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-JOIN"
>7.2.1.1. Joined Tables</A
></H3
><A
NAME="AEN3051"
></A
><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.
    </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 each combination of rows from
        <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> and
        <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
>, the derived 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
>,
        <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
></TT
>.  It is also 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).
       </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 <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 for them.
       </P
><P
>        <TT
CLASS="LITERAL"
>USING</TT
> is a shorthand notation: it takes a
        comma-separated list of column names, which the joined tables
        must have in common, and forms a join condition specifying
        equality of each of these pairs of columns.  Furthermore, the
        output of a <TT
CLASS="LITERAL"
>JOIN USING</TT
> has one column for each of
        the equated pairs of input columns, followed by all of the
        other columns from each table.  Thus, <TT
CLASS="LITERAL"
>USING (a, b,
        c)</TT
> is equivalent to <TT
CLASS="LITERAL"
>ON (t1.a = t2.a AND
        t1.b = t2.b AND t1.c = t2.c)</TT
> with the exception that
        if <TT
CLASS="LITERAL"
>ON</TT
> is used there will be two columns
        <TT
CLASS="LITERAL"
>a</TT
>, <TT
CLASS="LITERAL"
>b</TT
>, and <TT
CLASS="LITERAL"
>c</TT
> in the result,
        whereas with <TT
CLASS="LITERAL"
>USING</TT
> there will be only one of each.
       </P
><P
>        <A
NAME="AEN3135"
></A
>
        <A
NAME="AEN3138"
></A
>
        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 exactly those column names that appear in both
        input tables.  As with <TT
CLASS="LITERAL"
>USING</TT
>, these columns appear
        only once in the output table.
       </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 unconditionally 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 unconditionally 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
></DD
></DL
></DIV
><P
>     Joins of all types can be chained together or nested: either or
     both of <TT
CLASS="REPLACEABLE"
><I
>T1</I
></TT
> and
     <TT
CLASS="REPLACEABLE"
><I
>T2</I
></TT
> might 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
>     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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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"
>=&gt;</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>
    </P
></DIV
><DIV
CLASS="SECT3"
><H3
CLASS="SECT3"
><A
NAME="QUERIES-TABLE-ALIASES"
>7.2.1.2. Table and Column Aliases</A
></H3
><A
NAME="AEN3210"
></A
><A
NAME="AEN3213"
></A
><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 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 for the
     current query &mdash; it is no longer possible to refer to the table
     by the original name.  Thus:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM my_table AS m WHERE my_table.a &gt; 5;</PRE
><P>
     is not valid according to the SQL standard.  In
     <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> this will draw an error if the
     <A
HREF="runtime-config-compatible.html#GUC-ADD-MISSING-FROM"
>add_missing_from</A
> configuration variable is
     <TT
CLASS="LITERAL"
>off</TT
> (as it is by default).  If it is <TT
CLASS="LITERAL"
>on</TT
>,
     an implicit table reference will be added to the
     <TT
CLASS="LITERAL"
>FROM</TT
> clause, so the query is processed as if
     it were written as:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM my_table AS m, my_table AS my_table WHERE my_table.a &gt; 5;</PRE
><P>
     That will result in a cross join, which is usually not what you want.
    </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, using any of these forms, the alias hides the original
     names 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
><A
NAME="AEN3262"
></A
><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.  (See <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
><A
NAME="AEN3277"
></A
><A
NAME="AEN3279"
></A
><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 a table, view, or subquery column.
    </P
><P
>     If a table function returns a base data type, the single result
     column is named like the function. If the function returns a
     composite type, the result columns get the same names as the
     individual attributes of the type.
    </P
><P
>     A table function can be aliased in the <TT
CLASS="LITERAL"
>FROM</TT
> clause,
     but it also can be left unaliased. If a function is used in the
     <TT
CLASS="LITERAL"
>FROM</TT
> clause with no alias, the function name is used
     as the resulting table name.
    </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.  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 <TT
CLASS="LITERAL"
>dblink</TT
> function executes a remote query (see
     <TT
CLASS="FILENAME"
>contrib/dblink</TT
>).  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
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="QUERIES-WHERE"
>7.2.2. The <TT
CLASS="LITERAL"
>WHERE</TT
> Clause</A
></H2
><A
NAME="AEN3303"
></A
><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 (that is, if the result is
    false or null) it is discarded.  The search condition typically
    references at least some 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 &gt; 5</PRE
><P>
     and:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM a INNER JOIN b ON (a.id = b.id) WHERE b.val &gt; 5</PRE
><P>
     or perhaps even:
</P><PRE
CLASS="PROGRAMLISTING"
>FROM a NATURAL JOIN b WHERE b.val &gt; 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.  For
     outer joins there is no choice in any case: they must be done in
     the <TT
CLASS="LITERAL"
>FROM</TT
> clause.  An <TT
CLASS="LITERAL"
>ON</TT
>/<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 determines the addition
     of rows (for unmatched input rows) as well as the removal of rows
     from 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 &gt; 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 &gt; 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
><A
NAME="AEN3345"
></A
><A
NAME="AEN3347"
></A
><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 share 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 sharing common values into one group row that is
    representative of 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"
>=&gt;</SAMP
> <KBD
CLASS="USERINPUT"
>SELECT * FROM test1;</KBD
>
 x | y
---+---
 a | 3
 c | 2
 b | 5
 a | 1
(4 rows)

<SAMP
CLASS="PROMPT"
>=&gt;</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
    used in the grouping cannot be referenced except in aggregate
    expressions.  An example with aggregate expressions is:
</P><PRE
CLASS="SCREEN"
><SAMP
CLASS="PROMPT"
>=&gt;</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 9.18</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 on 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.  (Depending on how exactly the products
    table is set up, name and price might be fully dependent on the
    product ID, so the additional groupings could theoretically be
    unnecessary, but this is not implemented yet.)  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
>    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
><A
NAME="AEN3392"
></A
><P
>    If a table has been grouped using a <TT
CLASS="LITERAL"
>GROUP BY</TT
>
    clause, but then 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 a grouped
    table.  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"
>=&gt;</SAMP
> <KBD
CLASS="USERINPUT"
>SELECT x, sum(y) FROM test1 GROUP BY x HAVING sum(y) &gt; 3;</KBD
>
 x | sum
---+-----
 a |   4
 b |   5
(2 rows)

<SAMP
CLASS="PROMPT"
>=&gt;</SAMP
> <KBD
CLASS="USERINPUT"
>SELECT x, sum(y) FROM test1 GROUP BY x HAVING x &lt; '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 &gt; CURRENT_DATE - INTERVAL '4 weeks'
    GROUP BY product_id, p.name, p.price, p.cost
    HAVING sum(p.price * s.units) &gt; 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
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