Consider two relations P and Q. Let M is a primary key of relation Q and N is the foreign key of relation P. N refers to M. Let A be a set of all values of column M and B be a set of all non-NULL values of column N. Which of the following statement is correct ? (Suppose A and B holds in all instances of P and Q)
1
B ⊆ A
2
B ⊂ A
3
A ⊆ B
4
A ⊄ B and B ⊄ A
5
Question Not Attempted