check constraint on multiple tables?

Hi,

I've got this chain of tables:

ship --> (id_ship) --> cabin_type --> (id_cabin_type) --> cabin_category
--> (id_cabin_category) --> cabin

The 'cabin' table has (cabin_number, id_cabin_category ref. cabin_category)

How can I guarantee unicity of cabin_number per ship?

For now I added a unique(cabin_number,id_cabin_category) but this does
not guarantee unicity for (cabin_number,ship.id_ship).

What is the best solution? Adding an id_ship to 'cabin'? Or check'ing
with a join down to 'ship'? (if possible).

Thanks,