# Subquery expressions v16

SQL-compliant subquery expressions are available in EDB Postgres Advanced Server. All of these expression forms return Boolean `(TRUE/FALSE)`

results.

## EXISTS

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 that act as constants during any one evaluation of the subquery.

The subquery is generally executed only far enough to determine whether at least one row is returned, not all the way to completion. We recommend that you don't write a subquery that has any side effects (such as calling sequence functions). Whether the side effects occur might 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 `deptno`

, but it produces at most one output row for each `dept`

row, even though there are multiple matching `emp`

rows:

## IN

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).

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 is `NULL`

, not `FALSE`

. This is in accordance with SQL’s normal rules for Boolean combinations of null values.

As with `EXISTS`

, you can't assume that the subquery will evaluate completely.

## NOT IN

The right-hand side is a parenthesized subquery that 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.

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 is `NULL`

, not `TRUE`

. This is in accordance with SQL’s normal rules for Boolean combinations of null values.

As with `EXISTS`

, you can't assume that the subquery will evaluate completely.

## ANY/SOME

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`

.

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 is `NULL`

, not `FALSE`

. This is in accordance with SQL’s normal rules for Boolean combinations of null values.

As with `EXISTS`

, you can't assume that the subquery will evaluate completely.

## ALL

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 doesn't return `FALSE`

for any row and it returns `NULL`

for at least one row.

`NOT IN`

is equivalent to `<> ALL`

. As with `EXISTS`

, you can't assume that the subquery will evaluate completely.