Which of the following are true?
A. A database may have only one primary key
B. Every key is a candidate key
C. There can be multiple candidate keys
D. There can be more than one super keys
E. Every candidate key is a primary key.
Choose the correct answer from the options given below:

Explanation

A table can have only one primary key, so A is true. A table can have several minimal super keys (candidate keys), so C is true, and since any superset of a key is also a super key, there can be many super keys, so D is true. B is false because a general "key" need not be minimal, so not every key is a candidate key. E is false because only one candidate key is chosen as the primary key; the others remain candidate keys, not primary keys. Hence the true statements are A, C, D only.