Question

Say I have the following tables and columns:

comp: 
    id, 
    model

dvd: 
    id, 
    model

comp2dvd: 
    id, 
    id_comp, 
    id_dvd

A computer can have multiple dvd drives, even of the same model, and a dvd drive can appear in multiple computers. How do I make it so that comp2dvd table can have only existing comp and dvd ids?

Thanks

Answer 1

you cannot normally have a foreign key reference without an enforcing index which identifies a column or combination of columns as unique (such as, but not limited to, a primary key)

comp should have id as primary key

dvd should have id as primary key

comp2dvd should have id as primary key

comp2dvd should have id_comp as foreign key references(comp.id)

comp2dvd should have id_dvd as foreign key references(dvd.id)

DO NOT let comp2dvd have a unique index or constraint on the pair of columns (id_comp, id_dvd), since you need duplicates for computers with multiple identical drives