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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 long 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 side effects (such as calling sequence functions); whether the side effects occur might be unpredictable.

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 unimportant. 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 several
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 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
4.2.13. 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 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 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
4.2.13. 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 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 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
4.2.13. 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 case where the subquery
returns no rows). The result is NULL if no comparison with a
subquery row returns true, and at least one comparison returns
NULL.

See Section 9.23.5 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 case where the subquery returns no rows). The result is
"false" if any false result is found.
The result is NULL if no comparison with a subquery row returns
false, and at least one comparison returns NULL.

`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
4.2.13. 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 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 no comparison with a subquery
row returns false, and at least one comparison returns NULL.

See Section 9.23.5 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 4.2.13. 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 9.23.5 for details about the meaning of a row-wise comparison.