OCP-1Z0-051-V9.02-28题

28. Which two statements are true regarding constraints? (Choose two.)

A. A foreign key cannot contain NULL values. 

B. A column with the UNIQUE constraint can contain NULL values.

C. A constraint is enforced only for the INSERT operation on a table. 

D. A constraint can be disabled even if the constraint column contains data.

E. All constraints can be defined at the column level as well as the table level. 

Answer: BD

答案解析：

参考：http://blog.youkuaiyun.com/rlhua/article/details/12905109

A不对因为外键可以包含空值

C不对因为约束可以作用在INSERT,UPDATE,DELETE操作上

E不对因为NOT NULL约束只能在列级定义，不能在表级定义

B正确，唯一约束可以包含空值

D正确，即使约束列上有数据，也可以让约束不可用。