The answer you want to hear is a Check constraint, which is defined on a database table that limits the values entered into that column. These constraints are relatively easy to create, and they are the recommended type for enforcing domain integrity in SQL Server.
Triggers can also be used to restrict the values accepted in a field in a database table, but this solution requires the trigger to be defined on the table, which can hinder performance in certain situations. For this reason, Microsoft recommends Check constraints over all other methods for restricting domain integrity.

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