InterSystems SQL Reference
Checks a table for the existence of at least one corresponding row.
The EXISTS predicate tests a specified table, typically for existence of at least a row. Since the SELECT statement following the EXISTS is being checked for containing something, the clause is often of the form:
EXISTS (SELECT... FROM... WHERE...)
where a typical statement might be:
WHERE Table_B.Number = Table_A.Number)
In this example, the predicate tests for the existence of one or more rows specified by the subquery.
Note that the test must occur on a SELECT statement (not on a UNION).
The NOT EXISTS clause tests for the non-existence of a row in a table, as shown in the following example:
WHERE NOT EXISTS (SELECT * FROM BonusTable
WHERE NOT (BonusTable.Result = 'Positive'
AND Employees.EmployeeNum = BonusTable.EmployeeNum))
Where applicable, the system automatically applies Set-Valued Subquery Optimization (SVSO) to an EXISTS
or NOT EXISTS
subquery. For details on this optimization, and using the %NOSVSO keyword to override it, refer to Query Optimization Options on the FROM clause
Content Date/Time: 2019-08-22 06:48:58