What is the relationship between a superkey, a candidate key, and the primary key?

• A. A superkey is always minimal; a candidate key is any key; the primary key is never a candidate.
• B. A superkey uniquely identifies a row; a candidate key is a minimal superkey; the primary key is a chosen candidate key.
• C. A primary key is a superkey that is not a candidate key.
• D. A candidate key uniquely identifies a row; a superkey is a subset of a candidate key.

Answer: (B)

In relational databases, keys are about uniquely identifying rows. A superkey is any set of one or more attributes that guarantees a unique row. Among all the superkeys, a candidate key is a minimal one—no attribute can be dropped without losing the ability to identify each row uniquely. The primary key is a single candidate key that the database design chooses to be the main identifier for rows; there is only one primary key per table, and it is typically used for relations and foreign key references. So the idea is: every primary key is a candidate key, but not every candidate key becomes the primary key—the designer selects which candidate key will serve as the primary key.

Other descriptions fall short because a superkey doesn’t have to be minimal; it can include extra attributes beyond what's needed for uniqueness. The primary key is indeed a candidate key, not something outside that set. And a primary key is not “not a candidate key” but rather one specific candidate key that’s chosen. Also, a candidate key is not a subset of a superkey; rather, a superkey can include a candidate key plus additional attributes, since candidate keys are minimal forms of superkeys.