PostgreSQL 8.0.0rc2 Documentation
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# 9.16. 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.

## 9.16.1. `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 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);

## 9.16.2. `IN`

    expression IN (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 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.

    row_constructor IN (subquery)

The left-hand side of this form of `IN` is a row constructor, as described in Section 4.2.11. 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 special 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 row results are either unequal or null, with at least one null, then the result of `IN` is null.

## 9.16.3. `NOT IN`

    expression NOT IN (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 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.

    row_constructor NOT IN (subquery)

The left-hand side of this form of `NOT IN` is a row constructor, as described in Section 4.2.11. 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 special 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 row results are either unequal or null, with at least one null, then the result of `NOT IN` is null.

## 9.16.4. `ANY`/`SOME`

    expression operator ANY (subquery)
    expression operator SOME (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 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.

    row_constructor operator ANY (subquery)
    row_constructor operator SOME (subquery)

The left-hand side of this form of `ANY` is a row constructor, as described in Section 4.2.11. 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. Presently, only `=` and `<>` operators are allowed in row-wise `ANY` constructs. 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 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.

## 9.16.5. `ALL`

    expression operator ALL (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 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.

    row_constructor operator ALL (subquery)

The left-hand side of this form of `ALL` is a row constructor, as described in Section 4.2.11. 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. 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 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.

## 9.16.6. Row-wise Comparison

    row_constructor operator (subquery)

The left-hand side is a row constructor, as described in Section 4.2.11. 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. 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 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).

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