一、topN分析問題?

加偽列rownum行號(hào)的問題：

1：永遠(yuǎn)按照默認(rèn)的順序生成----在原表的順序下，跟排序無關(guān)

原因：臨時(shí)表的順序 create global temporary table****

特點(diǎn)：當(dāng)會(huì)話或者事務(wù)結(jié)束的時(shí)候，臨時(shí)表中的數(shù)據(jù)自動(dòng)刪除

2：rownum只能使用<,<=,不能使用>,>=

select? rownum2, empno, ename, sal from （select rownum,empno,ename,sal from emp order by sal ?desc）where rownum2<=3;

二、查詢薪水高于本部門平均工資的員工

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;

相關(guān)子查詢：把主查詢中的某些值作為參數(shù)傳遞給子查詢

select e.empno,e.name,e.sal,(select avg(sal) from emp where deptno=e.deptno )avgsal

from emp e

where e.sal> (select avg(sal) from emp where deptno=e.deptno )

三、統(tǒng)計(jì)每年入職的員工人數(shù)

select count(*) total ,

sum(if to_year(to_date(hiredate))='1981' ?then i=i+1 else )"1981",

from emp

行轉(zhuǎn)列：

wm_concat(varchar2)-->組函數(shù)

例：select deptno,wm_concat(ename) namelist

from emp

group by deptno;