OCP-1Z0-051-题目解析-第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

A：外键约束不能包括空值（错误，仅仅有非空约束和主键约束不能包括空值）。

B：唯一约束能够包括空值（正确，由于NULL<>NULL）

C：一个表上的约束仅仅对插入操作起作用（错误，Update、delete相同起作用）。

D：即使字段上有数据，该字段上的约束也能够被停用（正确）

E：全部的约束都能够定义在列级别和表级别（错误，Not Null仅仅能定义在列级别）。

时间： 2024-09-20 17:57:36

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