【2019年8月版】OCP 071認證考試原題-第33題

Choose three.

Which three statements are true about a self join?

A) It must be an inner join.

B) It can be an outer join.

C) The ON clause must be used.

D) It must be an equijoin.

E) The query must use twodifferent aliases for the table.

F) The ON clause can be used.

Answer：：BCD

(解析：左鏈接其實就是左外鏈接，是不一樣的稱呼而已，其結果都是同樣的，ANSI 語法有

左鏈接的語法，而 oracle 語法中用(+)來表示。)

select emp.empno,emp.deptno,dept.deptno,dept.dname

from emp left join dept on emp.deptno=dept.deptno;

select emp.empno,emp.deptno,dept.deptno,dept.dname

from emp left outer join dept on emp.deptno=dept.deptno;

select emp.empno,emp.deptno,dept.deptno,dept.dname

from emp ，dept

where emp.deptno=dept.deptno(+)