原文如下：
15. 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 synonym existing on the EMP table would have to be recreated.
B. Any constraints defined on the MGR_ID column would be removed by the above command.
C. Any views created on the EMP table that include the MGR_ID column would have to be dropped and
recreated.
D. Any index created on the MGR_ID column would continue to exist until the DROP UNUSED
COLUMNS command is executed.
Answer: B
虽然给出的答案是B，个人有些疑问。如果MGR_ID列是联合主键约束的一部分，以上语句就不能执行成功；而对于选项C，基于MGR_ID的视图确实出现错误，不能再使用。有没有高手能帮忙解释一下！
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是选B 。。
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觉得B的答案不够严谨 如果MGR_ID列是联合主键约束的一部分 是报错的