select e.ename,d.deptno,d.dname,d.loc

from dept d left join emp e 

on(d.deptno = e.deptno

and (e.deptno=10 or e.deptno=20))

order by 2;

另外，还可以先用emp.deptno进行筛选，然后进行外部联接：

select e.ename,d.deptno,d.dname,d.loc

from dept d

left join 

(select ename,deptno

from emp

where deptno in (10,20)

) e on (e.deptno =d.deptno)

order by 2;