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  • 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

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  • 原文地址:https://www.cnblogs.com/dylq/p/9885381.html
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