Sql server® 2012 t-sql fundamentals

Unique Constraints

A unique constraint enforces the uniqueness of rows, allowing you to implement the concept of 

alternate keys from the relational model in your database. Unlike with primary keys, you can define 

multiple unique constraints within the same table. Also, a unique constraint is not restricted to col-

umns defined as NOT NULL. According to standard SQL, a column with a unique constraint is sup-

posed to allow multiple NULL marks (as if two NULL marks were different from each other). However, 

SQL Server’s implementation rejects duplicate NULL marks (as if two NULL marks were equal to each 

other).

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