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题目

USER1.EMP has a referential integrity constraint defined on EMP.DNO that references USER1.DEPT.DNO.

USER1 executes these commads:

SQL> UPDATE dept SET loc='UK' where dno=1;

1 row updated.

SQL>UPDATE emp SET sal=1000 where eno=100;

1 row updated.

SQL>ALTER TABLE dept DROP(dno);

What will be the outcome and why?

A. It will fail because a referential integrity constraint is defined on USER1.EMP

B. It will execute successfully and drop DEPNT.DNO.

C. It will fail because there is an uncommitted transaction on USER1.DEPT.

D. It will execute successfully and drop the DEPT.DNO and EMP.DNO columns.

E. It will fail because there is an uncommitted transaction on USER1.EMP

答案

Answer：A

外键（FOREIGN KEY），引用完整性约束（a referential integrity constraint），指明一个列或者列的组合作为一个外键，并且在相同表或者不同表的主键或者唯一键和外键之间建立一个关系。前提是被引用的外键必须是另外一个表的主键。

SCOTT@OCPLHR1> ALTER TABLE dept DROP(deptno);

ALTER TABLE dept DROP(deptno)

ERROR at line 1:

ORA-12992: cannot drop parent key column