Is the Sql Server Unique Key also an Index?

sql-server indexing unique-key
Pure.Krome picture Pure.Krome · Dec 14, 2008 · Viewed 21.8k times · Source

I've got a column in a table (eg. UserName) which I want to make sure is unique. So I create a unique key for that column and call it IX_Users_UserName.

Now, if I do lots of searching for users based on their username I want to make sure there is an index for that field.

Do I need to create a separate index, or is the unique key also considered an index, just like the primary key is a clustered unique key?

Answer

tvanfosson picture tvanfosson · Dec 14, 2008

Unique Key: Unique Key enforces uniqueness of the column on which they are defined. Unique Key creates a non-clustered index on the column. Unique Key allows only one NULL Value.

Alter table to add unique constraint to column:

ALTER TABLE Authors ADD CONSTRAINT IX_Authors_Name UNIQUE(Name) GO

Source

More information from MSDN.

FWIW -- if your constraint doesn't create an index, I would avoid naming it IX_ as that would typically be assumed to be associated with one (IX = Index).

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