Referential Integrity
Let r1(R1) and r2(R2) be relations with primary key K1 and K2
A subset “a” of attributes R2 is a foreign key in relation r1, if for every tuple t2 in r2, there must be a tuple t1 in r1 such that t1[K1] = t2[a]
r1 Branch-scheme (branch_name, …
r2 Account-schema (account#, branch_name