25. choose the best answer
Evaluate the following SQL statement:
ALTER TABLE hr.emp
SET UNUSED (mgr_id);
Which statement is true regarding the effect of the above SQL statement?
A) Any views created on the EMP table that include the MGR_ID column would be automatically modified and remain valid.
B) Any constraints defined on the MGR_ID column would be removed by the above command.
C) Any synonym existing on the EMP table would have to be re-created.
D) Any index created on the MGR in column would continue to exist until the DROP UNUSED COLUMNS command is executed.
Answer：B
(注意：列一旦被设置为 unused 以后，基于该列的约束、索引都被删除，同时相关的视图被无法访问。)
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