OCP-1Z0-051-2015-20题

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
Explanation:

A正确，即使约束列上有数据，也可以让约束不可用disabled。

B不对 因为约束constraint可以作用在INSERT,UPDATE,DELETE操作上

C不对 因为外键foreign key 可以包含空值null values
D不对 因为NOT NULL约束只能在列级定义，不能在表级定义
E 正确，唯一约束UNIQUE constraint可以包含空值