Which statements are true regarding constraints?
Answer options
A
A column with the UNIQUE constraint can contain NULL values.
B
A foreign key cannot contain NULL values.
C
A constraint is enforced only for the INSERT operation.
D
A constraint cannot be disabled when the column contains data.
Correct answer: A column with the UNIQUE constraint can contain NULL values.
Explanation
UNIQUE constraints allow NULL values (a NULL is not considered equal to another NULL).