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# 9.22. Subquery Expressions

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.

## 9.22.1. `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);

## 9.22.2. `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.

## 9.22.3. `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.

## 9.22.4. `ANY`/`SOME`

expressionANY (operator)subqueryexpressionSOME (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_constructorANY (operator)subqueryrow_constructorSOME (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.

## 9.22.5. `ALL`

expressionALL (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_constructorALL (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.

## 9.22.6. Single-row 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.