OCP-1Z0-新051-61题版本-20

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：

参考：28：http://blog.csdn.net/rlhua/article/details/12921043