SQL Data Definition Language (DDL) 3

Question 1

Which constraint ensures a column cannot contain duplicate values but allows one NULL in many systems?

Accepted Answer

UNIQUE

Answer

A UNIQUE constraint forbids duplicate values while typically permitting a single NULL.

Question 2

What does a FOREIGN KEY constraint enforce?

Accepted Answer

Referential integrity between two tables

Answer

A FOREIGN KEY ensures values in one table match values in a referenced table, maintaining referential integrity.

Question 3

Which clause defines a column that automatically gets a value when none is supplied on INSERT?

Accepted Answer

DEFAULT

Answer

The DEFAULT clause provides a value automatically when an INSERT omits that column.

Question 4

A CHECK constraint is used to do what?

Accepted Answer

Limit the range of values allowed in a column

Answer

A CHECK constraint restricts column values to those satisfying a specified Boolean condition.

Question 5

How can you add a constraint to an existing table?

Accepted Answer

ALTER TABLE ... ADD CONSTRAINT

Answer

ALTER TABLE ... ADD CONSTRAINT attaches a new constraint to an already-existing table.

SQL Practice Test