Distinguish between a primary key and a unique constraint.

• A. Primary keys uniquely identify rows and cannot be null; unique constraints enforce uniqueness but can allow nulls depending on the database.
• B. Primary keys are used for data types; unique constraints are for indexing.
• C. Primary keys can contain duplicates; unique constraints cannot.
• D. Primary keys can be null; unique constraints cannot be null.

Answer: (A)

A primary key defines the main identity of each row: it must be unique and it cannot be null, making every row distinctly identifiable. This is why the primary key is used as the anchor for references from other tables (foreign keys) and is typically implemented as a NOT NULL unique index. A unique constraint, on the other hand, also enforces that values are unique across the constrained column(s), but it does not carry the same non-null requirement. Depending on the database, a unique constraint can allow nulls, because nulls are treated as unknown values and are not considered equal to each other. A table can have multiple unique constraints but only one primary key. So the best answer captures that the primary key uniquely identifies rows and cannot be null, while a unique constraint enforces uniqueness and may allow nulls.