Oracle Certificate Profession 1Z0-051-28

（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.

答案：（B、D）

解析：

NOT NULL約束只能被定義為列級約束

如果約束是定義在多列上的，那麼只能定義為表級約束

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