Although reference types are similar to foreign keys, there are significant differences.
In SQL:2003, referential integrity is maintained only by using a referential constraint
definition specified as part of the table definition. By themselves, reference types do not
provide referential integrity. Thus, the SQL reference type should not be confused with
that provided in the ODMG object model. In the ODMG model, OIDs are used to model
relationships between types and referential integrity is automatically defined, as discussed
in Section 27.2.2.