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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`

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 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 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 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_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 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 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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