
10.5 Adding Basic Constraints
Constraints are rules that restrict the population of the schema to allowable sets of data.
Previous chapters covered the ORM constraints, which are more comprehensive than
the constraints supported by Entity Relationship diagrams and source models. This sec-
tion addresses only four constraints, the Primary Key, Alternate Uniqueness, the
Mandatory (also called Not Null ) constraint, and the Foreign Key constraint.
Primary Key Constraint
The most fundamental constraint is the Primary Key constraint, which ensures that each
row in an entity is uniquely identifiable. Section 10.4 showed how to apply a primary
key constraint ...