# Composite primary key question

M Burke

Ranch Hand

Posts: 406

posted 10 years ago

With this senario, I am thinking that only D is incorrect. Because A alone cannot define the key. C may be wrong, but I think it depends on the value of B. I assume if a FK can be null, but it's a composite, that it would be invalid if one part of the key is null and the other is not. What do you think?

Which of the following statement(s) is/are not True __d______:

In relation R ( A, B, C, D, E ) , A and B together are defined as primary key, S ( X, A, B, Y, Z) is another relation, and the A and B in relation S are the same attributes A, B correspondently in R.

a. As A and B are defined as primary key in R, so the values of A and B in R cannot be null.

b. Attribute A in R itself cannot be null.

c. Attribute A in S may be null.

d. A in S can be defined as foreign key reference to A in R.

e. A and B in S together can be defined as foreign key reference to A and B in R.

Which of the following statement(s) is/are not True __d______:

In relation R ( A, B, C, D, E ) , A and B together are defined as primary key, S ( X, A, B, Y, Z) is another relation, and the A and B in relation S are the same attributes A, B correspondently in R.

a. As A and B are defined as primary key in R, so the values of A and B in R cannot be null.

b. Attribute A in R itself cannot be null.

c. Attribute A in S may be null.

d. A in S can be defined as foreign key reference to A in R.

e. A and B in S together can be defined as foreign key reference to A and B in R.

posted 10 years ago

Hmmm.. I didn't know such a law is present here.

Considering that this is not a real exam question,

'B' and 'C' can be true. D is not possible. That violates the derivative of 2NF. Having a foreign key which refers to part of a composite primary key is not advisable. (Now where did the smiley which shakes it's index finger go...)

Originally posted by Mark Spritzler:

We do not allow real questions to be posted and they do get deleted.

Mark

Hmmm.. I didn't know such a law is present here.

Considering that this is not a real exam question,

'B' and 'C' can be true. D is not possible. That violates the derivative of 2NF. Having a foreign key which refers to part of a composite primary key is not advisable. (Now where did the smiley which shakes it's index finger go...)

If you are not laughing at yourself, then you just didn't get the joke.