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oracle 约束A constraint in a table is defined with the INITIAL

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oracle 约束
A constraint in a table is defined with the INITIALLY IMMEDIATE clause.You executed the ALTER TABLE command with the ENABLE VALIDATE option to enable the constraint that was disabled.
What are the two effects of this command?(Choose two.)
A.It fails if any existing row violates the constraint.
B.It does not validate the existing data in the table.
C.It enables the constraint to be enforced at the end of each transaction.
D.It prevents insert,update,and delete operations on the table while the constraint is in the process of being enabled.
请高手解释下为何选择AD,
oracle 约束A constraint in a table is defined with the INITIAL
因为有VALIDATE选项,所以会对表中原有的数据进行检查,所以A、B两个选项中A是正确的;
因为约束是 INITIALLY IMMEDIATE,所以不会在提交事务时才应用约束,所以C是错误的;
约束enable的过程中,禁止所有的DML操作,这是默认的特性