Find the Cardinality of Natural Join

|X| represents number of tuples in X
bold letters represent keys in the relation

Consider the relations R(A, B) and S(A, C), and that R has a foreign key on A that references S.
|R ✶ S| (where ' * ' represents natural join) is:
The options are:
1. |R|
2. |S|
3. |R|.|S|
4. max(|R|, |S|)
5. min(|R|, |S|)

What I understand about the cardinality of natural join is that if there is no common attribute among the two relations then natural join will act like a cross-product and the cardinality will be r * s. But I don't understand how key constraints play a role in determining the cardinality. Can someone please explain?

Presuming the bold A in each schema means it is a key; and presuming the Foreign Key constraint holds -- that is, the A value for every row in R does correspond to an A value in S:

• Every row in R naturally joins to a row in S on A.
• There might be rows in S that don't join to R (because there's no Foreign Key constraint to enforce that).
• So the cardinality of the joined relations is the cardinality of R, answer 1.

Is there are real-life use for a schema like this? Consider S is Customer Name in C, keyed by Customer number in A. R holds date of birth in B, also keyed by Customer number in A. Every Customer must have a name; it's true every Customer (person) must have a d.o.b., but we don't need to record that unless/until they purchase age-restricted items.