This section describes the SQL-compliant subquery expressions available in Aster Database. All of the expression forms documented in this section return Boolean (true/false) results.
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 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; 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.
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).
Subquery SQL Expressions Aster Data proprietary and confidential
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. Note on SQL compliance: There is a small difference between a fully SQL-compliant implementation of IN/NOT IN and the Aster Database implementation. We can illustrate this with the example expression, “1 IN (<subquery>)”. Three cases are possible:
• Case 1: The subquery output contains 1. In this case both SQL-compliant implementations and Aster Database return true.
• Case 2: The subquery output does not contain 1, and does not contain a NULL. In this case, both SQL-compliant implementations and Aster Database return false.
• Case 3: The subquery output does not contain 1, but it does contain a NULL. In this case, an SQL-compliant implementation returns NULL, but the Aster Database implementation returns false.
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. See also the Note on SQL Compliance in the section above explaining “IN”.
ANY/SOME
expression operator ANY (subquery) expression operator SOME (subquery)
Used as an equality, ANY evaluates to TRUE if any one of a set of comparisons is TRUE. 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.
Aster Data proprietary and confidential Subquery SQL Expressions
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. 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.
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Window Functions
Window functions allow the calculation of aggregates and other values on a per-row basis as opposed to a per-set or per-group basis. Part of the SQL 2003 standard, window functions offer performance advantages for many OLAP applications that otherwise would have to calculate such values through other, less convenient, means. For example, a window function may be used to compute a running sum, a moving average, a delta between values in neighboring rows, as well as apply ranking and row numbering to a table.This section explains the different types of window functions and is divided into the following subsections: