MySQL常用语句

MySQL  SQL语句练习：1.建库建表--create database companycreate database company;use company;--create tables--部门表create table dept(deptno int(3) primary key,dname varchar(14),loc varchar(13));--雇员表create table emp(empno int(4) not null primary key,ename varchar(10),job varchar(10),mgr int(4),hiredate datetime,sal double,comm double,deptno int(3),foreign key(deptno) references dept(deptno));--工资级别表create table salgrade(grade int(3) primary key,losal int(3),hisal int(3)); ----------------------------------------------------------------------------------------------------------------------------------------2.插入数据（进行初始化）use company;--往部门表中查数据insert into dept values(10,'Accounting','New York');insert into dept values(20,'Research','Dallas');insert into dept values(30,'Sales','Chicago');insert into dept values(40,'Operations','Boston');insert into dept values(50,'Admin','Washing');--往雇员表中插数据insert into emp values(7369,'Smith','Clerk',7902,'1980-12-17',800,0,20);insert into emp values(7499,'Allen','Salesman',7698,'1981-2-20',1600,300,30);insert into emp values(7844,'Turner','Salesman',7499,'1981-9-8',1500,0,30);insert into emp values(7698,'Tom','Manager',0,'1981-9-8',6100,600,40);insert into emp values(7876,'Adams','Clerk',7900,'1987-5-23',1100,0,20);insert into emp values(7900,'James','Clerk',7698,'1981-12-3',2400,0,30);insert into emp values(7902,'Ford','Analyst',7698,'1981-12-3',3000,null,20);insert into emp values(7901,'Kik','Clerk',7900,'1981-12-3',1900,0,30);--往工资级别表中插数据insert into salgrade values(1,700,1200);insert into salgrade values(2,1201,1400);insert into salgrade values(3,1401,2000);insert into salgrade values(4,2001,3000);insert into salgrade values(5,3001,5000);insert into salgrade values(6,5001,10000);------------------------------------------------------------------------------------------------------------------------------------3.必会的5个组函数：max,min,avg,sum,count 要牢牢记住-----------------------------------------------------------------------------------------------------------------------------------4.练习：①查询雇员表中工资最高的雇员的员工号、员工姓名、工资和部门号。select empno,ename,sal,deptno from empwhere sal = (select max(sal) from emp);②单条查询语句综合练习题：薪水大于1200的雇员，按照部门编号进行分组，分组后的平均薪水必须大于1500，查询各分组的平均工资，按照工资的倒序进行排列。select avg(sal) avg_sal, deptnofrom empwhere sal > 1200group by deptnohaving avg_sal > 1500order by avg_sal desc;说明：此句基本上包含了SQL语句的子语句和排列顺序：select（要查询的字段）->from（从哪一张或哪几张表或视图）->where（过滤条件）->group by(having)（分组及条件）->order by（按哪个或哪几个字段进行升序或降序排列）。注意：SqlServer4.1中可能不支持在order语句中使用组函数avg，报错说：invalid use of group function（错误提示和现象有点对不上）解决办法：给avg(sal)起个别名avg_sal，这样在order语句中就直接使用这个别名③等值连接：查询每个雇员和其所在的部门名select ename,dname from emp,dept where (emp.deptno = dept.deptno);或者（推荐）(on中就写连接条件，where中就写过滤条件，各司其职)select ename,dname from emp join dept on(emp.deptno = dept.deptno);④非等值连接：查询每个雇员姓名及其工资所在的等级select ename,grade from emp e join salgrade s on(e.sal between s.losal and s.hisal);⑤查询雇员名第2个字母不是a的雇员的姓名、所在的组名、工资所在的等级。三张表的连接查询（先连接，再加上where语句进行过滤）select ename,dname,gradefrom emp e join dept d on(e.deptno = d.deptno)join salgrade s on(e.sal between s.losal and s.hisal)where ename not like '_a%';⑥查询每个雇员和其经理的姓名自连接：（事实上只有一张表，但把它当成两张表来用，使用别名来进行区分）select e1.ename,e2.ename from emp e1,emp e2 where (e1.mgr = e2.empno);或者：（推荐用join语句）select e1.ename,e2.ename from emp e1 join emp e2 on(e1.mgr = e2.empno);⑦查询每个雇员和其经理的姓名（包括公司老板本身（他上面没有经理））左外连接（会把左表中不符合连接条件的记录也显示出来）：select e1.ename,e2.ename from emp e1 left join emp e2 on(e1.mgr = e2.empno);⑧查询每个雇员的姓名及其所在部门的部门名（包括没有雇员的部门）右外连接（会把右表中不符合连接条件的记录也显示出来）：select ename,dname from emp e right join dept d on(e.deptno = d.deptno);⑨子查询1：查询每个部门中工资最高的人的姓名、薪水和部门编号先求出每个部门中的最高工资：select max(sal) max_sal,deptno from emp group by deptno在使用连接查询：select ename,sal,e.deptnofrom emp e join(select max(sal) max_sal,deptno from emp group by deptno) ton(e.sal = t.max_sal and e.deptno = t.deptno);⑩子查询2：查询每个部门平均工资所在的等级select deptno,avg_sal,grade from salgradejoin(select deptno,avg(sal) avg_sal from emp group by deptno) ton(t.avg_sal between salgrade.losal and salgrade.hisal);或者：select deptno,avg_sal,grade from(select deptno,avg(sal) avg_sal from emp group by deptno) tjoin salgrade s on(t.avg_sal between s.losal and s.hisal);⑪子查询3：查询每个部门内平均的薪水等级先求每个人的薪水等级select ename,deptno,grade from emp join salgrade s on(emp.sal between s.losal and s.hisal);再按组进行分组求平均select deptno,avg(grade) from(select ename,deptno,grade from emp join salgrade s on(emp.sal between s.losal and s.hisal)) tgroup by deptno;⑫子查询4：查询雇员中有哪些人是经理人：select ename from emp where empno in(select distinct mgr from emp);或者：select ename from emp join(select distinct mgr from emp) ton(emp.empno=t.mgr);⑬子查询5：不准用库函数，求雇员表中薪水的最高值。思路：两张完全相同的雇员表，左边一张，右边一张。拿左表中的薪水和右表中的薪水进行比较，左表中的最高薪水值必定不可能小于右表中的某一薪水值。先求出emp表中最高薪水以下的所有薪水值select distinct e1.sal from emp e1 join emp e2 on(e1.sal < e2.sal);不在此列的薪水值即为最高薪水值select distinct sal from emp where sal not in(select distinct e1.sal from emp e1 join emp e2 on(e1.sal < e2.sal));⑭子查询6：平均薪水最高的部门的部门编号　①：先求出每个部门的平均薪水和部门号（把这个看成一张表）select avg(sal) avg_sal,deptno from emp group by deptno;　②：再求出平均薪水最高值（把这个看成一个值）select max(avg_sal) from (select avg(sal) avg_sal,deptno from emp group by deptno) t;　③：对①表使用②条件进行查询即可select avg_sal,deptno from (select avg(sal) avg_sal,deptno from emp group by deptno) t1where avg_sal= (select max(avg_sal) from (select avg(sal) avg_sal,deptno from emp group by deptno) t2);说明:在Oracle中可以使用组函数嵌套来稍微简化SQL语句的复杂程度（最多是两层嵌套）：select max(avg(sal)) from emp group by deptno;来求出平均薪水最高值注意：这种组函数嵌套在MySQL中不可以用⑮子查询7：求平均薪水最高的部门的部门名称（在子查询6的基础之上）说明：在实际应用中，达到这种程度的SQL语句已经算比较复杂了。select dname from dept where deptno = ( select deptno from     (select avg(sal) avg_sal,deptno from emp group by deptno) t1  where avg_sal=     (select max(avg_sal) from (select avg(sal) avg_sal,deptno from emp group by deptno) t2));-------------------------------------------------------------------------------------------------------------------------------------------------⑯子查询8：查询平均薪水的等级最低的部门名称说明：在实际应用中，很少使用到如此复杂的SQL语句。但要掌握一步步的思路，本例是为了锻炼逻辑能力。①求平均薪水select avg(sal) avg_sal,deptno from emp group by deptno;②求平均薪水的等级select avg_sal,deptno,grade from  (select avg(sal) avg_sal,deptno from emp group by deptno) tjoin salgrade s on(t.avg_sal between s.losal and hisal);③求平均薪水的等级最低的那个值select min(grade) from(  select avg_sal,deptno,grade from      (select avg(sal) avg_sal,deptno from emp group by deptno) t   join salgrade s on(t.avg_sal between s.losal and hisal)) t3;④平均薪水的等级最低的部门的部门编号(将②和③组合起来，将②看成要查询的表，将③看成查询条件)select deptno from(   select avg_sal,deptno,grade from       (select avg(sal) avg_sal,deptno from emp group by deptno) t    join salgrade s on(t.avg_sal between s.losal and hisal)) t2where grade=(   select min(grade) from    (   select avg_sal,deptno,grade from           (select avg(sal) avg_sal,deptno from emp group by deptno) t            join salgrade s on(t.avg_sal between s.losal and hisal)    ) t3);⑤平均薪水的等级最低的部门名称select dname from dept where deptno=(  select deptno from     (   select avg_sal,deptno,grade from             (select avg(sal) avg_sal,deptno from emp group by deptno) t         join salgrade s on(t.avg_sal between s.losal and hisal)     ) t2   where grade=      (   select min(grade) from         (   select avg_sal,deptno,grade from                    (select avg(sal) avg_sal,deptno from emp group by deptno) t                 join salgrade s on(t.avg_sal between s.losal and hisal)      ) t3      ));或者：还有种思路，使用join连接多张表（效果同上面的④和⑤，但比上面的稍微简单些）④查询平均薪水的等级最低的部门的部门名、部门编号、平均工资和等级(将②和③组合起来，将②和dept表join起来，将③看成查询条件)select dname,t2.deptno,avg_sal,grade from(   select avg_sal,deptno,grade from      (select avg(sal) avg_sal,deptno from emp group by deptno) t    join salgrade s on(t.avg_sal between s.losal and hisal)) t2join dept on(t2.deptno=dept.deptno)where t2.grade=(   select min(grade) from    (   select avg_sal,deptno,grade from           (select avg(sal) avg_sal,deptno from emp group by deptno) t            join salgrade s on(t.avg_sal between s.losal and hisal)    ) t3);说明：本例中有个待改进的地方，就是有重复的语句段（出现了两次）：select avg_sal,deptno,grade from   (select avg(sal) avg_sal,deptno from emp group by deptno) tjoin salgrade s on(t.avg_sal between s.losal and hisal)改进1：在Oracle中使用创建视图，可以简化上面的SQL语句（视图也是一张表，不过是虚表（实际数据仍然存在表emp和表salgrade中）、子查询）：create view v$_dept_avg_sal_info asselect avg_sal,deptno,grade from   (select avg(sal) avg_sal,deptno from emp group by deptno) tjoin salgrade s on(t.avg_sal between s.losal and hisal);将上例简化如下：select dname,t2.deptno,avg_sal,grade fromv$_dept_avg_sal_info t2join dept on(t2.deptno=dept.deptno)where t2.grade=(   select min(grade) from v$_dept_avg_sal_info);改进2：MySQL5.0中创建视图(MySQL5新特点(视图))：待运行验证：create view v_dept_avg_sal_info asselect avg_sal,deptno,grade from   (select avg(sal) avg_sal,deptno from emp group by deptno) tjoin salgrade s on(t.avg_sal between s.losal and hisal);将上例简化为：select dname,v2.deptno,avg_sal,grade fromv_dept_avg_sal_info v2join dept on(v2.deptno=dept.deptno)where v2.grade=(   select min(grade) from v_dept_avg_sal_info);