1、分頁查詢

--查詢員工表中(5,8]頁的記錄
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.查詢員工表中薪水大於本部門平均薪水的員工

--多表查詢 
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)

--相關子查詢   
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語句前加  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

 再執行命令 select * from table(dbms_xplan.display) 檢視兩種方法的資源佔用大小。最後對比發現，相關子查詢更好

3、統計每個年份入職員工人數的和

--方法一 函式
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;

--慕課學習筆記