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`

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);

`IN`

IN (`expression`

)`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.

IN (`row_constructor`

)`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.

```
NOT
IN
```

NOT IN (`expression`

)`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.

NOT IN (`row_constructor`

)`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.

`ANY`

/`SOME`

`expression`

ANY (`operator`

)`subquery`

`expression`

SOME (`operator`

)`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_constructor`

ANY (`operator`

)`subquery`

`row_constructor`

SOME (`operator`

)`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 the comparison does not return true
for any row, and it returns NULL for at least one row.See Section 9.23.5 for details about the meaning of a row constructor comparison.

`ALL`

`expression`

ALL (`operator`

)`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 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_constructor`

ALL (`operator`

)`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 the comparison does not
return false for any subquery row, and it returns NULL for at least
one row.See Section 9.23.5 for details about the meaning of a row constructor comparison.

`row_constructor`

(`operator`

)`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 constructor comparison.

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