A System.Data.ForeignKeyConstraint restricts the action performed when a value in a column (or columns) is either deleted or updated. Such a constraint is intended to be used with primary key columns. In a parent/child relationship between two tables, deleting a value from the parent table can affect the child rows in one of the following ways.

• The child rows can also be deleted (a cascading action).

• The values in the child column (or columns) can be set to null values.

• The values in the child column (or columns) can be set to default values.

• An exception can be generated.

A System.Data.UniqueConstraint becomes active when attempting to set a value in a primary key to a non-unique value.