Transact-SQL provides keywords for maintaining entity integrity, which ensures that a value is supplied for every column that requires one, and domain integrity, which ensures that each value in a column belongs to the set of legal values for that column.
Defaults and rules define the integrity constraints that are used during data entry and modification. A default is a value linked to a particular column or datatype, and inserted by the system if no value is provided during data entry. Rules are user-defined integrity constraints linked to a particular column or datatype, and enforced at data entry time.