之前只对单张表进行查询,但在实际开发中,数据往往来自不同的表,所以需要多表查询,在写这篇博客之前,先用之前的一张表做一个简单回顾。
使用的是scott数据库,里面包含三张表, dept , emp , salgradeemp。
select * from emp where (sal>500 or job='MANAGER') and ename like 'J%';
select * from emp order by deptno,sal desc;
select ename,sal*12+ifnull(comm,0) as '年薪' from emp order by 年薪 desc;
mysql> select ename,job from emp where sal = (select max(sal) from emp);
mysql> select ename,sal from emp where sal > (select avg(sal) from emp);
mysql> select deptno,format(avg(sal),2),max(sal) from emp group by deptno;
mysql> select deptno,avg(sal) from emp group by deptno having avg(sal) < 2000;
mysql> select job,count(*),format(avg(sal),2) from emp group by job;
数据往往来源于多张表,因此需要联合查询。
mysql> select emp.ename,emp.sal,dept.dname from emp,dept where emp.deptno = dept.deptno;
mysql> select ename,sal,dname from emp,dept where dept.deptno = emp.deptno and dept.deptno = 10;
mysql> select ename,sal,grade from emp,salgrade where emp.sal between losal and hisal;
自连接是指在同一张表连接查询。
显示员工FORD的上级领导的编号和姓名(mgr是员工领导的编号–empno)
select empno,ename from emp where emp.empno = (select mgr from emp where ename = 'FORD');
--使用到表的别名
mysql> select leader.empno,leader.ename from emp leader,emp worker where leader.empno = worker.mgr and worker.ename = 'FORD';
子查询是指嵌入在其它SQL语句中的select语句,也叫嵌套查询。
select * from emp where deptno = (select deptno from emp where ename = 'SMITH');
select ename,job,sal,deptno from emp where job in (select distinct job from emp where deptno = 10) and deptno<>10;
select ename,sal,deptno from emp where sal > all (select sal from emp where deptno = 30);
mysql> select ename,sal,deptno from emp where sal > any(select sal from emp where deptno = 30);
单行子查询是指返回单列单行数据,多行子查询是指返回单列多行数据,而多列子查询是指返回多个列数据的子查询语句。
mysql> select ename from emp where (deptno,job) = (select deptno,job from emp where ename = 'SMITH') and ename<>'SMITH';
mysql> select ename,deptno,sal,format(asal,2) from emp,(select avg(sal) asal,deptno dt from emp group by deptno) tmp
-> where emp.sal > tmp.asal and emp.deptno = tmp.dt;
mysql> select emp.ename,emp.sal,emp.deptno,ms from emp,(select max(sal) ms,deptno from emp group by deptno) tmp
-> where emp.deptno = tmp.deptno and emp.sal = tmp.ms;
mysql> select dept.dname,dept.deptno,dept.loc,count(*) '部门人数' from emp,dept
-> where emp.deptno = dept.deptno
-> group by dept.deptno,dept.dname,dept.loc;
mysql> select dept.deptno,dname,mycnt,loc from dept,(select count(*) mycnt,deptno from emp group by deptno) tmp
-> where dept.deptno = tmp.deptno;
在实际应用中,为了合并多个select的执行结果,可以使用集合操作符union,union all。
union:取得两个结果的并集,当使用该操作符时,会自动去掉结果集中的重复行。
mysql> select ename,sal,job from emp where sal > 2500 union
-> select ename,sal,job from emp where job = 'MANAGER';
union all:取得两个结果集的并集,当使用该操作符时,不会去掉结果集中的重复行。
mysql> select ename,sal,job from emp where sal > 2500 union all
-> select ename,sal,job from emp where job = 'MANAGER';