QUESTION 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 re-created.

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
验证过程：我在soctt用户下复制emp表为tt1表，在tt1表上进行做验证。

SQL> show user;
USER is "SCOTT"

SQL> create table tt1 as select * from emp;                           ----创建tt1表
Table created.

SQL> desc tt1;
 Name                                                                                Null?    Type
 ----------------------------------------------------------------------------------- -------- --------------------------------------------------------
 EMPNO                                                                                        NUMBER(4)
 ENAME                                                                                        VARCHAR2(10)
 JOB                                                                                          VARCHAR2(9)
 MGR                                                                                          NUMBER(4)
 HIREDATE                                                                                     DATE
 SAL                                                                                          NUMBER(7,2)
 COMM                                                                                         NUMBER(7,2)
 DEPTNO                                                                                       NUMBER(2)

SQL> alter table tt1 add constraint empno_pk primary key(empno);               ----在empno列上添加主键
Table altered.

SQL> select TABLE_NAME,CONSTRAINT_NAME,CONSTRAINT_TYPE,STATUS from user_constraints where table_name='TT1';
TABLE_NAME                     CONSTRAINT_NAME                C    STATUS
------------------------------ ------------------------------ - ---     -----
TT1                                         EMPNO_PK                             P     ENABLED

SQL> select index_name from user_indexes where table_name='TT1';            ----tt1表生产了索引
INDEX_NAME
------------------------------
EMPNO_PK

SQL> create or replace view tt1_view  as select empno,ename,sal from tt1;      ----在tt1表上创建视图tt1_view
View created.
SQL> select * from tt1_view;
     EMPNO ENAME             SAL
---------- ---------- ----------
      7369 SMITH             902
      7499 ALLEN            1702
      7521 WARD             1352
      7566 JONES            3077
      7654 MARTIN           1352
      7698 BLAKE            2952
      7782 CLARK            2552
      7788 SCOTT            3102
      7839 KING             5102
      7844 TURNER           1602
      7876 ADAMS            1202
      7900 JAMES            1052
      7902 FORD             3102
      7934 MILLER           1402
14 rows selected.

SQL> show user;                                                 ----在sys用户下为scott.tt1创建同义词
USER is "SYS"
SQL> create synonym tt1 for scott.tt1;
Synonym created.

SQL> show user;                                                ---在scott用户下执行命令 alter table tt1 set unused column empno
USER is "SCOTT"
SQL> alter table tt1 set unused column empno;
Table altered.

验证选项B：查看tt1表在empno上的主键

SQL> select TABLE_NAME,CONSTRAINT_NAME,CONSTRAINT_TYPE,STATUS from user_constraints where table_name='TT1';

no rows selected

查询结果看出定义在empno上的主键已经被删除。


验证选项C：查看在tt1表上创建的视图tt1_view（包含empno列）

SQL> select * from tt1_view;
select * from tt1_view
              *
ERROR at line 1:
ORA-04063: view "SCOTT.TT1_VIEW" has errors（视图出错，但未被删除）

验证选项D：查看在tt1.emp上索引是否存在

SQL> select index_name from user_indexes where table_name='TT1';

no rows selected

查询结果看出索引已经被删除。

验证选项A：在sys用户下查看同义词

SQL> show user;
USER is "SYS"
SQL> select * from tt1;
ENAME      JOB              MGR HIREDATE         SAL       COMM     DEPTNO
---------- --------- ---------- --------- ---------- ---------- ----------
SMITH      CLERK           7902 17-DEC-80        902                    20
ALLEN      SALESMAN        7698 12-MAR-07       1702                    30
WARD       SALESMAN        7698 22-FEB-81       1352        500         30
JONES      MANAGER         7839 02-APR-81       3077                    20
MARTIN     SALESMAN        7698 28-SEP-81       1352       1400         30
BLAKE      MANAGER         7839 01-MAY-81       2952                    30
CLARK      MANAGER         7839 09-JUN-81       2552                    10
SCOTT      ANALYST         7566 19-APR-87       3102                    20
KING       PRESIDENT            17-NOV-81       5102                    10
TURNER     SALESMAN        7698 08-SEP-81       1602          0         30
ADAMS      CLERK           7788 23-MAY-87       1202                    20
JAMES      CLERK           7698 03-DEC-81       1052                    30
FORD       ANALYST         7566 03-DEC-81       3102                    20
MILLER     CLERK           7782 23-JAN-82       1402                    10
14 rows selected.

查询结果看出表tt1上的同义词还能正常使用，不用重建。

总结：命令alter table  xxx  set  unuserd  ***
致使在该列上创建的约束和索引都会被立即删除；
导致包含该列的view出错，但不会被删除；
但为该表建立的synonym还会正常使用。
注意：该命令是不可逆操作，某列一旦被set unused，就不会再恢复。