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This section describes the SQL-compliant subquery expressions available in PostgreSQL. All of the expression forms documented in this section return Boolean (true/false) results.

EXISTS (subquery)

The argument of `EXISTS` is an
arbitrary `SELECT` statement, or
*subquery*. The subquery is evaluated to
determine whether it returns any rows. If it returns at least
one row, the result of `EXISTS` is
"true"; if the subquery returns no
rows, the result of `EXISTS` is
"false".

The subquery can refer to variables from the surrounding query, which will act as constants during any one evaluation of the subquery.

The subquery will generally only be executed far enough to determine whether at least one row is returned, not all the way to completion. It is unwise to write a subquery that has any side effects (such as calling sequence functions); whether the side effects occur or not may be difficult to predict.

Since the result depends only on whether any rows are
returned, and not on the contents of those rows, the output
list of the subquery is normally uninteresting. A common coding
convention is to write all `EXISTS`
tests in the form `EXISTS(SELECT 1 WHERE
...)`. There are exceptions to this rule however, such as
subqueries that use `INTERSECT`.

This simple example is like an inner join on `col2`, but it produces at most one output row for
each `tab1` row, even if there are
multiple matching `tab2` rows:

SELECT col1 FROM tab1 WHERE EXISTS(SELECT 1 FROM tab2 WHERE col2 = tab1.col2);

expressionIN (subquery)

The right-hand side is a parenthesized subquery, which must
return exactly one column. The left-hand expression is
evaluated and compared to each row of the subquery result. The
result of `IN` is "true" if any equal subquery row is found. The
result is "false" if no equal row is
found (including the special case where the subquery returns no
rows).

Note that if the left-hand expression yields null, or if
there are no equal right-hand values and at least one
right-hand row yields null, the result of the `IN` construct will be null, not false. This is in
accordance with SQL's normal rules for Boolean combinations of
null values.

As with `EXISTS`, it's unwise to assume
that the subquery will be evaluated completely.

row_constructorIN (subquery)

The left-hand side of this form of `IN`
is a row constructor, as described in Section
4.2.11. The right-hand side is a parenthesized subquery,
which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery
result. The result of `IN` is "true" if any equal subquery row is found. The
result is "false" if no equal row is
found (including the special case where the subquery returns no
rows).

As usual, null values in the rows are combined per the
normal rules of SQL Boolean expressions. Two rows are
considered equal if all their corresponding members are
non-null and equal; the rows are unequal if any corresponding
members are non-null and unequal; otherwise the result of that
row comparison is unknown (null). If all the per-row results
are either unequal or null, with at least one null, then the
result of `IN` is null.

expressionNOT IN (subquery)

The right-hand side is a parenthesized subquery, which must
return exactly one column. The left-hand expression is
evaluated and compared to each row of the subquery result. The
result of `NOT IN` is "true" if only unequal subquery rows are found
(including the special case where the subquery returns no
rows). The result is "false" if any
equal row is found.

Note that if the left-hand expression yields null, or if
there are no equal right-hand values and at least one
right-hand row yields null, the result of the `NOT IN` construct will be null, not true. This is
in accordance with SQL's normal rules for Boolean combinations
of null values.

As with `EXISTS`, it's unwise to assume
that the subquery will be evaluated completely.

row_constructorNOT IN (subquery)

The left-hand side of this form of `NOT
IN` is a row constructor, as described in Section
4.2.11. The right-hand side is a parenthesized subquery,
which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery
result. The result of `NOT IN` is
"true" if only unequal subquery rows
are found (including the special case where the subquery
returns no rows). The result is "false" if any equal row is found.

As usual, null values in the rows are combined per the
normal rules of SQL Boolean expressions. Two rows are
considered equal if all their corresponding members are
non-null and equal; the rows are unequal if any corresponding
members are non-null and unequal; otherwise the result of that
row comparison is unknown (null). If all the per-row results
are either unequal or null, with at least one null, then the
result of `NOT IN` is null.

expressionoperatorANY (subquery)expressionoperatorSOME (subquery)

The right-hand side is a parenthesized subquery, which must
return exactly one column. The left-hand expression is
evaluated and compared to each row of the subquery result using
the given `operator`, which must
yield a Boolean result. The result of `ANY` is "true" if any
true result is obtained. The result is "false" if no true result is found (including
the special case where the subquery returns no rows).

`SOME` is a synonym for `ANY`. `IN` is equivalent to
`= ANY`.

Note that if there are no successes and at least one
right-hand row yields null for the operator's result, the
result of the `ANY` construct will be
null, not false. This is in accordance with SQL's normal rules
for Boolean combinations of null values.

As with `EXISTS`, it's unwise to assume
that the subquery will be evaluated completely.

row_constructoroperatorANY (subquery)row_constructoroperatorSOME (subquery)

The left-hand side of this form of `ANY` is a row constructor, as described in Section
4.2.11. The right-hand side is a parenthesized subquery,
which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery
result, using the given `operator`. The result of `ANY` is "true" if the
comparison returns true for any subquery row. The result is
"false" if the comparison returns
false for every subquery row (including the special case where
the subquery returns no rows). The result is NULL if the
comparison does not return true for any row, and it returns
NULL for at least one row.

See Section 9.17.5 for details about the meaning of a row-wise comparison.

expressionoperatorALL (subquery)

The right-hand side is a parenthesized subquery, which must
return exactly one column. The left-hand expression is
evaluated and compared to each row of the subquery result using
the given `operator`, which must
yield a Boolean result. The result of `ALL` is "true" if all
rows yield true (including the special case where the subquery
returns no rows). The result is "false" if any false result is found. The result
is NULL if the comparison does not return false for any row,
and it returns NULL for at least one row.

`NOT IN` is equivalent to `<> ALL`.

As with `EXISTS`, it's unwise to assume
that the subquery will be evaluated completely.

row_constructoroperatorALL (subquery)

The left-hand side of this form of `ALL` is a row constructor, as described in Section
4.2.11. The right-hand side is a parenthesized subquery,
which must return exactly as many columns as there are
expressions in the left-hand row. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery
result, using the given `operator`. The result of `ALL` is "true" if the
comparison returns true for all subquery rows (including the
special case where the subquery returns no rows). The result is
"false" if the comparison returns
false for any subquery row. The result is NULL if the
comparison does not return false for any subquery row, and it
returns NULL for at least one row.

See Section 9.17.5 for details about the meaning of a row-wise comparison.

row_constructoroperator(subquery)

The left-hand side is a row constructor, as described in Section 4.2.11. The right-hand side is a parenthesized subquery, which must return exactly as many columns as there are expressions in the left-hand row. Furthermore, the subquery cannot return more than one row. (If it returns zero rows, the result is taken to be null.) The left-hand side is evaluated and compared row-wise to the single subquery result row.

See Section 9.17.5 for details about the meaning of a row-wise comparison.