Table 1: telephone manufacturers ID Name 001 Sony 002 Lucent 003 Motorola Table 2: telephone models PK ID Name Company ID 001 001 KZY123 001 002 002 KZY345 001 003 003 ZZ9 001 004 001 987Z 002 005 002 988X 002 006 001 Mr. Phone 003 007 002 Portaphone 003Whay, because one of the requirements in the relational model is that any attribute (field) must be atomic. That measn that the field cannot be broken down further and still have meaning. Your compound key is not atomic, you can break it aprat and the two pieces have meaning without each other. Break them apart and make them two distinct fields. As for the PK of the child table, I have a personla preference for using surrogate PKs, but you can easily create and index on CompanyID + Id to produce a primary key for the child table.