57、(14-17) choose two:
Examine the structure of the DEPARTMENTS table
You execute the following command:
SQL> ALTER TABLE departments
SET UNUSED (country);
Which two statements are true?
A) A new column, COUNTRY, can be added to the DEPARTMENTS table after executing the command.
B) Indexes created on the COUNTRY column exist until the DROP UNUSED COLUMNS command is executed.
C) Unique key constraints defined on the COUNTRY column are removed.
D) Synonyms existing on the DEPARTMENTS table would have to be re-created.
E) Views created on the DEPARTMENTS table that include the COUNTRY column are automatically
modified and remain valid.
Answer：AC
(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。经过实验，可以马上往该表添加名字一样的列，有关 set unsed 的知识点考题
前面也有出现过，本题一个特点就是答案 A)