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:
SELECT name FROM Table_A WHERE EXISTS (SELECT * FROM Table_B 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:
SELECT EmployeeName,Age FROM Employees 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 reference page.