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Differences between UNIQUE Constraint and Index



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Add Date: 01/27/2013
In this article we will learn about Difference between Unique Constraint and Index.
The differences between UNIQUE constraint and index are quite subtle since UNIQUE constraint in SQL Server is implemented as UNIQUE index.

UNIQUE constraint is a database object. UNIQUE index is not a database object. UNIQUE constraint creates a UNIQUE (candidate) KEY which can be used for Foreign Key referencing according to ANSI SQL rules.

T-SQL allows FK referencing to UNIQUE index columns also, however, this is not an ANSI feature.

The usage syntax is different. For example, index cannot be created in CREATE TABLE. Differences vis-a-vis table variables as demonstrated following.

Related system commands are different:


Happy Programming!!!

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