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### Postgres as a Service

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2.4.12 Subquery ExpressionsThis section describes the SQL-compliant subquery expressions available in Advanced Server. All of the expression forms documented in this section return Boolean (true/false) results.2.4.12.1 EXISTSThe 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.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:2.4.12.2 INThe 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.2.4.12.3 NOT INThe 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.2.4.12.4 ANY/SOMEThe 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).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.2.4.12.5 ALLThe 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 does not return false for any row, and it returns null for at least one row.NOT IN is equivalent to <> ALL. As with EXISTS, it’s unwise to assume that the subquery will be evaluated completely.