A constraint line in the physical data model is the equivalent of a parent/child relation line in an ERD; it is specified as identifying or non-identifying. Neither the non-specific relationship nor the super-sub relationship of an ERD exist in a physical data model.

The constraint line in the PDM is a visual representation of the foreign key constraints in the database schema. The foreign key constraints control referential integrity in the database, ensuring, for example, that a row in the parent table will not be deleted if there are corresponding rows in the child table.

An identifying constraint indicates that an instance of the parent table identifies an instance of the child table; that is, if there is no PARENT table there can be no corresponding CHILD TABLE. The primary keys of the parent table are foreign key components of the child table, pictured on the diagram as [PK] [FK].

Non-identifying constraints are used when an instance of the parent table does not identify a corresponding instance of the child table. The primary key of the parent table is propagated to the child table as [FK], that is, it is a foreign key but not primary key.

Information about the relationship between the parent and child tables is entered through the Constraint definition editor editor.