OCP题库解析：11g-1Z0-051-v9.02-028题

OCP题库解析：11g-1Z0-051-v9.02-028题，完整题库请点击这里联系老师咨询了解

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

试题解析：

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。