方式1(多表查詢):
select e.empno,e.ename ,e.sal ,d.salVal from emp e,(
select deptno ,avg(sal) salVal from emp group by deptno) d
where e.deptno=d.deptno and e.sal > d.salVal;
方式二(相關子查詢):
select d.empno,d.ename,d.sal
(
select avg(sal) from emp where dempno =d.dempno
) avgsal
from emp d
where d.sal > (select avg(sal) from emp p where p.dempno = d.dempno)
xplain plan for 一般放在執行sql開頭,執行查詢計劃;
select * from (dbms_xplan.display);可以打印查看執行計劃,一般放在末尾,然後查看耗費多少cpu的執行資源,結果發現使用相關子查詢比多表查詢耗費cpu資源要少。