ocp題庫解析：11g-1Z0-051-v9.02-028題
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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
試題解析：
A不對因為外鍵可以包含空值
C不對因為約束可以作用在INSERT,UPDATE,DELETE操作上
E不對因為NOT NULL約束只能在列級定義，不能在表級定義
B正確，唯一約束可以包含空值
D正確，即使約束列上有數據，也可以讓約束不可用。
