2.11. Subquery Expressions

This section describes the SQL-compliant subquery expressions available in Hyper. All of the expression forms documented in this section return Boolean results.

2.11.1. 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. For this reason, it is unwise to write a subquery that has side effects, since those side effects may or may not occur depending on the results of the subquery.

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 expressions in the form EXISTS(SELECT 1 WHERE ...). However, there are exceptions to this rule, such as subqueries that use INTERSECT. In that case, a SELECT 1 in all operands will cause the EXISTS expression to always evaluate to true. The same holds for EXCEPT, with the EXISTS evaluating to false in that case.

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);

2.11.2. 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 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.

2.11.3. 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 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.

2.11.4. 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 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.

2.11.5. 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 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.