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UNIQUE Constraints in Microsoft SQL Server
UNIQUE constraints allow SQL Server administrators to specify that a column may not contain duplicate values. When you create a new UNIQUE constraint, SQL Server checks the column in question to determine whether it contains any duplicate values. If the table contains preexisting duplicates, the constraint creation command fails. Similarly, once you have a UNIQUE constraint on a column, attempts to add or modify data that would cause duplicates to exist also fail.

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