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Does UNIQUE KEY supports more than one NULL Value?

By Ashok Nalam on 12 Dec 2012 | Category: SQL Server | Tagged: Constraints unique key 
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No. In SQL server we can insert only one NULL value to column which has UNIQUE KEY constraint.

Example:

CREATE TABLE SamepleUnique
(
Id INT PRIMARY KEY,
Name varchar(20) UNIQUE
)
INSERT INTO SamepleUnique VALUES(1,'Ram')
INSERT INTO SamepleUnique VALUES(2,'Krish')
INSERT INTO SamepleUnique VALUES(3,Null)
INSERT INTO SamepleUnique VALUES(4,Null) --Error as Violation of UNIQUE KEY constraint 'UQ__SamepleU__737584F603317E3D'. Cannot insert duplicate key in object 'dbo.SamepleUnique'.
--The statement has been terminated.


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Comments (1)
 
1. By Karna on 28 Dec 2012

Uniuqe Key--Accepts any value exactly once in a column including Null Primary Key--Accepts any value exactly once in a column excluding Null