Database MCQ — 15 Practice Questions with Answers

Explanation

A SELECT query that retrieves table columns normally identifies the source table in the FROM clause, and SQL identifiers with spaces must be delimited. Option D omits the FROM clause and also writes last name as two unquoted tokens. Therefore D is the erroneous statement.

Explanation

A DELETE statement removes rows that satisfy its WHERE clause; if the WHERE clause is absent, all rows in the table are deleted and the table remains empty. Therefore DELETE FROM EMPLOYEE; deletes every one of the n tuples from EMPLOYEE. Hence C is correct.

Explanation

In logic, an implication P₁ → P₂ is false only when P₁ is true and P₂ is false. Therefore it is equivalent to ¬P₁ ∨ P₂, which is option A. The other forms change the truth condition of implication.

Explanation

A materialized view stores a query result physically, so it must be maintained when the base tables change. A trigger can fire automatically on insert, update, or delete events and apply the required maintenance logic. Pointers or clone objects do not provide this automatic view-maintenance mechanism.

Explanation

A candidate key is defined as a minimal superkey: it uniquely identifies a tuple, and no proper subset of it can still be a superkey. Primary keys are selected from candidate keys, while foreign keys reference keys in another relation. Therefore minimal superkeys are candidate keys, so C is correct.