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 far 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 any side effects (such as
calling sequence functions); whether the side effects occur or not
may 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 `col2`, but
it produces at most one output row for each `tab1` row,
even if there are multiple matching `tab2` rows:

SELECT col1 FROM tab1
WHERE EXISTS(SELECT 1 FROM tab2 WHERE col2 = tab1.col2);

*expression* IN (*value*[, ...])

The right-hand side of this form of `IN` is a parenthesized list
of scalar expressions. The result is "true" if the left-hand expression's
result is equal to any of the right-hand expressions. This is a shorthand
notation for

*expression* = *value1*
OR
*expression* = *value2*
OR
...

Note that if the left-hand expression yields null, or if there are
no equal right-hand values and at least one right-hand expression 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.

**Note: ** This form of `IN` is not truly a subquery expression, but it
seems best to document it in the same place as subquery `IN`.

*expression* IN (*subquery*)

The right-hand side of this form of `IN` 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).

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.

(*expression* [, *expression* ...]) IN (*subquery*)

The right-hand side of this form of `IN` is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. 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 special
case where the subquery returns no rows).

As usual, null values in the expressions or subquery 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 row results are either unequal or null, with at least one null,
then the result of `IN` is null.

*expression* NOT IN (*value*[, ...])

The right-hand side of this form of `NOT IN` is a parenthesized list
of scalar expressions. The result is "true" if the left-hand expression's
result is unequal to all of the right-hand expressions. This is a shorthand
notation for

*expression* <> *value1*
AND
*expression* <> *value2*
AND
...

Note that if the left-hand expression yields null, or if there are
no equal right-hand values and at least one right-hand expression yields
null, the result of the `NOT IN` construct will be null, not true
as one might naively expect.
This is in accordance with SQL's normal rules for Boolean combinations
of null values.

**Tip: ** `x NOT IN y` is equivalent to `NOT (x IN y)` in all
cases. However, null values are much more likely to trip up the novice when
working with `NOT IN` than when working with `IN`.
It's best to express your condition positively if possible.

*expression* NOT IN (*subquery*)

The right-hand side of this form of `NOT IN` 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 special 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.

(*expression* [, *expression* ...]) NOT IN (*subquery*)

The right-hand side of this form of `NOT IN` is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. 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 special case where the subquery returns no rows).
The result is "false" if any equal row is found.

As usual, null values in the expressions or subquery 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 row results are either unequal or null, with at least one null,
then the result of `NOT IN` is null.

*expression* *operator* ANY (*subquery*)
*expression* *operator* SOME (*subquery*)

The right-hand side of this form of `ANY` 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`.

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.

(*expression* [, *expression* ...]) *operator* ANY (*subquery*)
(*expression* [, *expression* ...]) *operator* SOME (*subquery*)

The right-hand side of this form of `ANY` is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery result,
using the given *operator*. Presently,
only `=` and `<>` operators are allowed
in row-wise `ANY` queries.
The result of `ANY` is "true" if any equal or unequal row is
found, respectively.
The result is "false" if no such row is found (including the special
case where the subquery returns no rows).

As usual, null values in the expressions or subquery 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 there is at least one null row result, then the result of `ANY`
cannot be false; it will be true or null.

*expression* *operator* ALL (*subquery*)

The right-hand side of this form of `ALL` 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.

`NOT IN` is equivalent to `<> ALL`.

Note that if there are no failures but at least one right-hand row yields
null for the operator's result, the result of the `ALL` 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.

(*expression* [, *expression* ...]) *operator* ALL (*subquery*)

The right-hand side of this form of `ALL` is a parenthesized
subquery, which must return exactly as many columns as there are
expressions in the left-hand list. The left-hand expressions are
evaluated and compared row-wise to each row of the subquery result,
using the given *operator*. Presently,
only `=` and `<>` operators are allowed
in row-wise `ALL` queries.
The result of `ALL` is "true" if all subquery rows are equal
or unequal, respectively (including the special
case where the subquery returns no rows).
The result is "false" if any row is found to be unequal or equal,
respectively.

As usual, null values in the expressions or subquery 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 there is at least one null row result, then the result of `ALL`
cannot be true; it will be false or null.

(*expression* [, *expression* ...]) *operator* (*subquery*)
(*expression* [, *expression* ...]) *operator* (*expression* [, *expression* ...])

The left-hand side is a list of scalar expressions. The right-hand side
can be either a list of scalar expressions of the same length, or a
parenthesized subquery, which must return exactly as many columns as there
are expressions on the left-hand side. 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, or to the right-hand expression list.
Presently, only `=` and `<>` operators are allowed
in row-wise comparisons.
The result is "true" if the two rows are equal or unequal, respectively.

As usual, null values in the expressions or subquery 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 the row comparison is unknown (null).