There are foreign keys between the parent and child tables. In some cases the referential constraints cannot be kept via foreign keys due to technical limitations, so the stored procedures and triggers perform the constraints.
Due to a mutating problem, some referential integrity in Oracle and SQL Server is kept only in stored procedures and triggers or in the application (referred to in the table as o(p)).
Some parent tables in , mostly supporting tables (also known as validation tables) have initial rows for cases where the child table is not required to have a defined value for the parent table. The ID number of these rows is usually 0. This emulation is done by the application or the database code and a restricted foreign key exists.