What is a unique index?

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What is a unique index?

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A unique index specifies that the value of a column (or columns), upon which the index is based, must be unique. If a user creates a unique index containing several columns, the unique index ensures that the combination of all values contained in the index is unique. Both clustered and nonclustered indexes can be unique provided that the data in the column is unique. Hence, both a unique clustered index and multiple unique nonclustered indexes on the same table can be created. When a primary key or unique constraint is created on the specified columns, a unique index is automatically created.


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