最近学习Oracle,老师讲了三个比较典型的问题,做一下总结,也便于以后复习.
下图显示的是三个题的题干和要查询的结果:
===========================================第一题============================
找到员工表中工资最高的前三名:
先说明一个现象:
1 SQL> select rownum,ename from emp; 2 3 ROWNUM ENAME 4 ---------- ---------- 5 1 SMITH 6 2 ALLEN 7 3 WARD 8 4 JONES 9 5 MARTIN 10 6 BLAKE 11 7 CLARK 12 8 SCOTT 13 9 KING 14 10 TURNER 15 11 ADAMS 16 12 JAMES 17 13 FORD 18 14 MILLER
如果在加上一些条件限制:
1 SQL> select rownum ,ename,sal from emp order by sal desc; 2 3 ROWNUM ENAME SAL 4 ---------- ---------- ---------- 5 9 KING 5000 6 13 FORD 3000 7 8 SCOTT 3000 8 4 JONES 2975 9 6 BLAKE 2850 10 7 CLARK 2450 11 2 ALLEN 1600 12 10 TURNER 1500 13 14 MILLER 1300 14 3 WARD 1250 15 5 MARTIN 1250 16 11 ADAMS 1100 17 12 JAMES 950 18 1 SMITH 800
加上order by rownum 依旧没有变化,说明rownum是基于原始表emp进行排序的 ,固定住了.
把这个查询的结果集再看成一个新表select ename,sal from emp order by sal desc
用rownum就会对这个新表进行排序.排序的rownum按照查询的顺序来展示.
select rownum,ename,sal from (select ename,sal from emp order by sal desc) where rownum < 4 ROWNUM ENAME SAL ---------- ---------- ---------- 1 KING 5000 2 SCOTT 3000 3 FORD 3000
===========================================第二题===================================
找到员工表中薪水大于本部门平均薪水的员工
SQL> 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; EMPNO ENAME SAL AVGSAL ---------- ---------- ---------- ---------- 7499 ALLEN 1600 1566.66667 7566 JONES 2975 2175 7698 BLAKE 2850 1566.66667 7788 SCOTT 3000 2175 7839 KING 5000 2916.66667 7902 FORD 3000 2175
e.deptno = empavg.deptno and e.sal > empavg.salavg
翻译下来就是:第一个条件是本部门的员工,第二个是薪水大于部门的平均薪水
还有一种不用group by分组函数也能查询到
叫:相关子查询
相关子查询:将主查询中的某些值作为参数传递给子查询.
相关子查询对外边的主表有一个要求,必须有一个别名.
先不看上面结果的第四列(AVGSAL),可以很容易的查询到
select empno,ename, sal from emp e
然后查询某个部门的平均工资:
select avg(sal) from emp where deptno = ?
需要确定这个deptno是什么!
结合上面两个语句
select empno,ename, sal from emp e
where e.sal >( select avg(sal) from emp where deptno = ?)
这个问好?应该代表的是查询的当前员工所在的部门号传给这个问号.
变成:
select empno,ename, sal from emp e
where e.sal > (select avg(sal) from emp where deptno = e.deptno);
然后主查询列上要加上所在部门平均工资
把上面的最后一句查询部门的平均工资拷贝到主查询列上.就可以了.
SQL> select empno,ename, 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); EMPNO ENAME SAL AVGSAL ---------- ---------- ---------- ---------- 7499 ALLEN 1600 1566.66667 7566 JONES 2975 2175 7698 BLAKE 2850 1566.66667 7788 SCOTT 3000 2175 7839 KING 5000 2916.66667 7902 FORD 3000 2175
===============================================第三题=======================
统计每年入职的员工的个数!
select count(*) from emp where decode(substr(to_char(hiredate,‘yyyy-mm-dd‘),3,2)80 decode(substr(to_char(hiredate,"yyyy-mm-dd"),3,4),‘81‘,count+1) SQL> select count(*) , 2 sum(decode(to_char(hiredate,‘RR‘),‘80‘,1,0)) "1980", 3 sum(decode(to_char(hiredate,‘RR‘),‘81‘,1,0)) "1981", 4 sum(decode(to_char(hiredate,‘RR‘),‘87‘,1,0)) "1987" 5 from emp; COUNT(*) 1980 1981 1987 ---------- ---------- ---------- ---------- 14 1 10 2