Exercise 19.4 Assume that you are given a relation with attributes ABCD. Assume that no re...

Exercise 19.4 Assume that you are given a relation with attributes ABCD.

1. Assume that no record has NULL values. Write an SQL query that checks whether the functional dependency A → B holds.
2. Assume again that no record has NULL values. Write an SQL assertion that enforces the functional dependency A → B.
3. Let us now assume that records could have NULL values. Repeat the previous two questions under this assumption.