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Exists Predicate

Wed, 03/04/2009 - 20:02 |  hdanzl

Specify a test for a non-empty set.


Syntax

<exists predicate> ::= EXISTS <table subquery>


Notes

♦ The predicate evaluates to TRUE if the cardinality of the subquery result is greater than 0, else to FALSE.


Examples

1) The following example selects students enrolled in more than 2 courses:

       SELECT studentID, studentName

       FROM students s

       WHERE EXISTS (

SELECT COUNT( * )

FROM enrolls

WHERE studentID = s.studentID

HAVING COUNT( * ) > 2

       )

2) The following example selects students not enrolled in any courses:

       SELECT studentID, studentName

       FROM students s

       WHERE EXISTS (

SELECT COUNT( * )

FROM enrolls

WHERE studentID = s.studentID

HAVING COUNT( * ) = 0

       )


Conformance

SQL:2003 standard

-

-

Core SQL

Feature T501 "Enhanced EXISTS predicate"
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