Referencial Integrity with Merges

Referencial Integrity with Merges

I have two tables:


TABLE ONE:
product_id
product_name

TABLE TWO:
product_id
product_name


And I have One Merge:


MERGE TABLE:
product_id
product_name


All of which are in MyISAM.


My Question is, how do i keep the product_id index in the Merge table to be unique?

Basically,


Is there a way which I can have (referring to PHP here):

OBJECT ONE
- OBJECT TWO
- OBJECT THREE

Where Object Two and Three EXTEND Object One

and object two and three both have unique ID numbers in the DB.