ASSORTED

Question 1

Discuss the various reasons that lead to the occurrence of NULL values in relations

Answer 1

In general, we can have several meanings for null values, such as “value unknown,” “value exists but is not available,” or “attribute does not apply to this tuple.”

Question 2

Consider a relation R (A, B, C, D, E) with the following dependencies:
AB→C
CD→E
DE→B
Is AB a candidate key of this relation? If not, is ABD? Explain your answer

Answer 2

No. The closure of AB does not give you all of the attributes of the relation.
Yes, ABD is a candidate key. No subset of its attributes is a key.