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Constraints are used by SQL Server to enforce column data integrity.
Both primary and foreign keys are forms of constraints.
Other forms of constraints used for a column include:
UNIQUE constraints enforce uniqueness within a table on non-primary key columns.
DEFAULT constraints can be used when you do not know the value of a column in a row when it is first inserted into a table.
CHECK constraints are used to define the data format and values allowed for a column.
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