QUESTION NO: 20
Which two statements are true regarding constraints? (Choose two.)
A. A constraint can be disabled even if the constraint column contains data
B. A constraint is enforced only for the INSERT operation on a table
C. A foreign key cannot contain NULL values
D. All constraints can be defined at the column level as well as the table level
E. A columns with the UNIQUE constraint can contain NULL values
Answer: A,E
答案解析：