Oracle高级查询案例

1、分页查询

--查询员工表中(5,8]页的记录
select rownum,r,empno,ename,sal from (
select rownum r , empno,ename,sal
from (select rownum,empno,ename,sal from emp order by sal desc) e1
where rownum<=8) e2
where  r > 5

 

2.查询员工表中薪水大于本部门平均薪水的员工

--多表查询 
select e.empno,e.ename,e.sal,d.avgsal
from emp e,(select deptno,avg(sal) avgsal from emp group by deptno) d
where e.deptno = d.deptno and e.sal>d.avgsal
select * from table(dbms_xplan.display)

--相关子查询   
select empno,ename,sal,(select avg(sal) avgsal from emp where deptno = e.deptno)
from emp e
where sal > (
	select avg(sal) avgsal from emp where deptno = e.deptno
)

 注意：在SQL语句前加  explain plan for

explain plan for
select e.empno,e.ename,e.sal,d.avgsal
from emp e,(select deptno,avg(sal) avgsal from emp group by deptno) d
where e.deptno = d.deptno and e.sal>d.avgsal

 再执行命令 select * from table(dbms_xplan.display) 查看两种方法的资源占用大小。最后对比发现，相关子查询更好

 

3、统计每个年份入职员工人数的和

--方法一 函数
select count(*) total ,
sum(decode(to_char(hiredate,'YYYY'),'1980',1,0)) "1980",
sum(decode(to_char(hiredate,'YYYY'),'1981',1,0)) "1981",
sum(decode(to_char(hiredate,'YYYY'),'1982',1,0)) "1982",
sum(decode(to_char(hiredate,'YYYY'),'1987',1,0)) "1987"
from emp;

--方法二
select 
	(select count(*) from emp) Total,
	(select count(*) from emp where to_char(hiredate,'YYYY')='1980') "1980" ,
	(select count(*) from emp where to_char(hiredate,'YYYY')='1981') "1981" ,
	(select count(*) from emp where to_char(hiredate,'YYYY')='1982') "1982" ,
	(select count(*) from emp where to_char(hiredate,'YYYY')='1987') "1987" 
from dual;

 

--慕课学习笔记