[SQL] Query

[Pedro Igor]

Regards .....     I hope someone can help me in this query.   Have a field in a table that needs to check if another table has the value that is being inserted.   Ex:     table A       - id int constraint pkey_id primary key,              table B       - id int constraint fkey_A_B references A,       Here comes my doubt:       table C       - id int constraint fkey_A_C references A check (if exists B.id = C.id)       How can i build this expression so, when I insert a tupple in table C the field will check in the table A(ok, because is a foreign key) and also in table B ....     I have tried : check (select count(b.id) from B b where b.id = id) <> 0) .... but doesnīt work ......     I can use trigger here, but i donīt know if is the best solution .....          Thanks,           Pedro Igor