Specify a test for the absence of duplicate rows.

 

Syntax

 

<unique predicate> ::= UNIQUE <table subquery>

 

 

Notes

 

The predicate evaluates to TRUE if the subquery result is empty or it contains no duplicate rows, else to FALSE.



 

 

Examples

 

1)

The following example selects students enrolled in none or only 1 course:



 


SELECT studentID, studentName


FROM students s


WHERE UNIQUE (




SELECT studentID


FROM enrolls


WHERE studentID = s.studentID




)



 

 


 

Conformance

 

SQL:2003 standard

-

Feature F291 "UNIQUE predicate"