文章目录*1.基本查询回顾* 2.多表查询* 3.自连接* 4.子查询* * 4.1单行子查询 * 4.2多行子查询 * 4.3多列子查询 * 4.4在from子句中使用子查询 * 4.5合并查询 * * 4.5.1 union * 4.5.2 union all1.基本查询回顾--------表的内容如下 mysql select * from emp; ±-------±-------±----------±-----±--------------------±--------±--------±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±-------±----------±-----±--------------------±--------±--------±------- | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | ±-------±-------±----------±-----±--------------------±--------±--------±------- 14 rows in set (0.00 sec) mysql select * from dept; ±-------±-----------±--------- | deptno | dname | loc | ±-------±-----------±--------- | 10 | ACCOUNTING | NEW YORK | | 20 | RESEARCH | DALLAS | | 30 | SALES | CHICAGO | | 40 | OPERATIONS | BOSTON | ±-------±-----------±--------- 4 rows in set (0.00 sec) mysql select * from salgrade; ±------±------±------ | grade | losal | hisal | ±------±------±------ | 1 | 700 | 1200 | | 2 | 1201 | 1400 | | 3 | 1401 | 2000 | | 4 | 2001 | 3000 | | 5 | 3001 | 9999 | ±------±------±------ 5 rows in set (0.00 sec) * 查询工资高于500或岗位为MANAGER的雇员同时还要满足他们的姓名首字母为大写的J // 使用模糊查询 select * from emp where (sal500 or job‘MANAGER’) and ename like ‘J%’; // 使用函数 select * from emp where (sal500 or job‘MANAGER’) and substring(ename,1,1)‘J’; mysql select * from emp where (sal500 or job‘MANAGER’) and ename like ‘J%’; ±-------±------±--------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±--------±-----±--------------------±--------±-----±------- | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | ±-------±------±--------±-----±--------------------±--------±-----±------- 2 rows in set (0.00 sec) mysql select * from emp where (sal500 or job‘MANAGER’) and substring(ename,1,1)‘J’; ±-------±------±--------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±--------±-----±--------------------±--------±-----±------- | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | ±-------±------±--------±-----±--------------------±--------±-----±------- 2 rows in set (0.00 sec)* 按照部门号升序而雇员的工资降序排序 select * from emp order by deptno asc, sal desc; mysql select * from emp order by deptno asc,sal desc; ±-------±-------±----------±-----±--------------------±--------±--------±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±-------±----------±-----±--------------------±--------±--------±------- | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | ±-------±-------±----------±-----±--------------------±--------±--------±-------* 使用年薪进行降序排序 年薪等于工资*12奖金 需要对奖金进行判断如果奖金为null则奖金为0 select ename, sal12ifnull(comm,0) as ‘年薪’ from emp order by 年薪 desc; mysql select ename,sal12ifnull(comm,0) as ‘年薪’ from emp order by 年薪 desc; ±-------±--------- | ename | 年薪 | ±-------±--------- | SMITH | 9600.00 | | ALLEN | 19500.00 | | WARD | 15500.00 | | JONES | 35700.00 | | MARTIN | 16400.00 | | BLAKE | 34200.00 | | TEST | 29400.00 | | SCOTT | 36000.00 | | KING | 60000.00 | | TURNER | 18000.00 | | ADAMS | 13200.00 | | JAMES | 11400.00 | | FORD | 36000.00 | | MILLER | 15600.00 | ±-------±--------- 14 rows in set (0.00 sec)* 显示工资最高的员工的名字和工作岗位 这里使用分组查询即可先查出最高的工资然后查询工资等于最高工资的员工的姓名和工作岗位 select ename,job from emp where sal (select max(sal) from emp); mysql select ename,job from emp where sal (select max(sal) from emp); ±------±---------- | ename | job | ±------±---------- | KING | PRESIDENT | ±------±---------- 1 row in set (0.00 sec)* 显示工资高于平均工资的员工信息 这里使用分组查询即可 select ename,sal from emp where sal (select avg(sal) from emp); mysql select ename,sal from emp where sal (select avg(sal) from emp); ±------±-------- | ename | sal | ±------±-------- | JONES | 2975.00 | | BLAKE | 2850.00 | | TEST | 2450.00 | | SCOTT | 3000.00 | | KING | 5000.00 | | FORD | 3000.00 | ±------±-------- 6 rows in set (0.00 sec)* 显示每个部门的平均工资和最高工资 select deptno,avg(sal),max(sal) from emp group by deptno; mysql select deptno,avg(sal),max(sal) from emp group by deptno; ±-------±------------±--------- | deptno | avg(sal) | max(sal) | ±-------±------------±--------- | 10 | 2425.000000 | 5000.00 | | 20 | 2175.000000 | 3000.00 | | 30 | 1690.000000 | 2850.00 | ±-------±------------±--------- 3 rows in set (0.00 sec)* 显示平均工资低于2000的部门号和它的平均工资 select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal 2000; mysql select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal 2000; ±-------±------------ | deptno | avg_sal | ±-------±------------ | 30 | 1690.000000 | ±-------±------------ 1 row in set (0.00 sec)* 显示每种岗位的雇员总数平均工资 select job,count(), avg(sal) from emp group by job; mysql select job,count(), avg(sal) from emp group by job; ±----------±---------±------------ | job | count() | avg(sal) | ±----------±---------±------------ | ANALYST | 2 | 3000.000000 | | CLERK | 4 | 1037.500000 | | MANAGER | 3 | 2758.333333 | | PRESIDENT | 1 | 5000.000000 | | SALESMAN | 4 | 1400.000000 | ±----------±---------±------------ 5 rows in set (0.00 sec)2.多表查询------实际开发中往往数据来自不同的表所以需要多表查询。本节我们用一个简单的公司管理系统有三张表emp,dept,salgrade来演示如何进行多表查询。案例显示雇员名、雇员工资以及所在部门的名字因为上面的数据来自emp和dept表因此要联合查询其实我们只要emp表中的deptno dept表中的deptno字段的记录 select ename,sal,dname from emp,dept where emp.deptnodept.deptno; mysql select ename,sal,dname from emp,dept where emp.deptnodept.deptno; ±-------±--------±----------- | ename | sal | dname | ±-------±--------±----------- | SMITH | 800.00 | RESEARCH | | ALLEN | 1600.00 | SALES | | WARD | 1250.00 | SALES | | JONES | 2975.00 | RESEARCH | | MARTIN | 1250.00 | SALES | | BLAKE | 2850.00 | SALES | | CLACK | 2450.00 | ACCOUNTING | | SCOTT | 3000.00 | RESEARCH | | KING | 5000.00 | ACCOUNTING | | TURNER | 1500.00 | SALES | | ADAMS | 1100.00 | RESEARCH | | JAMES | 950.00 | SALES | | FORD | 3000.00 | RESEARCH | | MILLER | 1300.00 | ACCOUNTING | ±-------±--------±----------- 14 rows in set (0.00 sec)显示部门号为10的部门名员工名和工资 mysql select dname,ename,sal from emp,dept where emp.deptnodept.deptno and dept.deptno10; ±-----------±-------±-------- | dname | ename | sal | ±-----------±-------±-------- | ACCOUNTING | CLACK | 2450.00 | | ACCOUNTING | KING | 5000.00 | | ACCOUNTING | MILLER | 1300.00 | ±-----------±-------±-------- 3 rows in set (0.00 sec)* 显示各个员工的姓名工资及工资级别 mysql select ename,sal,grade from emp,salgrade where sal between losal and hisal; mysql select ename,sal,grade from emp,salgrade where sal between losal and hisal; ±-------±--------±------ | ename | sal | grade | ±-------±--------±------ | SMITH | 800.00 | 1 | | ALLEN | 1600.00 | 3 | | WARD | 1250.00 | 2 | | JONES | 2975.00 | 4 | | MARTIN | 1250.00 | 2 | | BLAKE | 2850.00 | 4 | | CLACK | 2450.00 | 4 | | SCOTT | 3000.00 | 4 | | KING | 5000.00 | 5 | | TURNER | 1500.00 | 3 | | ADAMS | 1100.00 | 1 | | JAMES | 950.00 | 1 | | FORD | 3000.00 | 4 | | MILLER | 1300.00 | 2 | ±-------±--------±------ 14 rows in set (0.00 sec)3.自连接-----自连接是指在同一张表连接查询案例显示员工FORD的上级领导的编号和姓名mgr是员工领导的编号–empno使用的子查询select ename,empno from emp where empno(select mgr from emp where ename‘FORD’);使用多表查询自查询select e2.ename,e2.empno from emp e1,emp e2 where e1.ename‘FORD’ and e1.mgre2.empno; mysql select e1.ename,e2.empno from emp e1,emp e2 where e1.ename‘FORD’ and e1.mgre2.empno; ±------±------- | ename | empno | ±------±------- | FORD | 007566 | ±------±------- 1 row in set (0.00 sec)4.子查询-----子查询是指嵌入在其他sql语句中的select语句也叫嵌套查询### 4.1单行子查询返回一行记录的子查询*显示SMITH同一部门的员工select * from emp where deptno(select deptno from emp where ename‘SMITH’); mysql select * from emp where deptno(select deptno from emp where ename‘SMITH’); ±-------±------±--------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±--------±-----±--------------------±--------±-----±------- | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | ±-------±------±--------±-----±--------------------±--------±-----±------- 5 rows in set (0.00 sec)### 4.2多行子查询返回多行记录的子查询*in关键字查询和10号部门的工作岗位相同的雇员的名字岗位工资部门号但是不包含10自己的 select ename,job,sal,deptno from emp where job in(select job from emp where deptno10) and deptno10; mysql select ename,job,sal,deptno from emp where job in(select job from emp where deptno10) and deptno10; ±------±--------±--------±------- | ename | job | sal | deptno | ±------±--------±--------±------- | JONES | MANAGER | 2975.00 | 20 | | BLAKE | MANAGER | 2850.00 | 30 | | SMITH | CLERK | 800.00 | 20 | | ADAMS | CLERK | 1100.00 | 20 | | JAMES | CLERK | 950.00 | 30 | ±------±--------±--------±------- 5 rows in set (0.00 sec) *all关键字显示工资比部门30的所有员工的工资高的员工的姓名、工资和部门号 // 使用聚合函数 select ename,sal,deptno from emp where sal(select max(sal) from emp where deptno30); mysql select ename,sal,deptno from emp where sal(select max(sal) from emp where deptno30); ±------±--------±------- | ename | sal | deptno | ±------±--------±------- | JONES | 2975.00 | 20 | | SCOTT | 3000.00 | 20 | | KING | 5000.00 | 10 | | FORD | 3000.00 | 20 | ±------±--------±------- 4 rows in set (0.01 sec) // 使用all关键子 select ename,sal,deptno from emp where salall(select sal from emp where deptno30); mysql select ename,sal,deptno from emp where salall(select sal from emp where deptno30); ±------±--------±------- | ename | sal | deptno | ±------±--------±------- | JONES | 2975.00 | 20 | | SCOTT | 3000.00 | 20 | | KING | 5000.00 | 10 | | FORD | 3000.00 | 20 | ±------±--------±------- 4 rows in set (0.00 sec) *any关键字显示工资比部门30的任意员工的工资高的员工的姓名、工资和部门号包含自己部门的员工 // 使用聚合函数 mysql select ename,sal,deptno from emp where sal (select min(sal) from emp where deptno30) and deptno30; ±-------±--------±------- | ename | sal | deptno | ±-------±--------±------- | JONES | 2975.00 | 20 | | CLACK | 2450.00 | 10 | | SCOTT | 3000.00 | 20 | | KING | 5000.00 | 10 | | ADAMS | 1100.00 | 20 | | FORD | 3000.00 | 20 | | MILLER | 1300.00 | 10 | ±-------±--------±------- 7 rows in set (0.00 sec) // 使用any关键字 mysql select ename,sal,deptno from emp where sal any(select sal from emp where deptno30) and deptno30; ±-------±--------±------- | ename | sal | deptno | ±-------±--------±------- | JONES | 2975.00 | 20 | | CLACK | 2450.00 | 10 | | SCOTT | 3000.00 | 20 | | KING | 5000.00 | 10 | | ADAMS | 1100.00 | 20 | | FORD | 3000.00 | 20 | | MILLER | 1300.00 | 10 | ±-------±--------±------- 7 rows in set (0.00 sec) ### 4.3多列子查询单行子查询是指子查询只返回单列单行数据多行子查询是指返回单列多行数据都是针对单列而言的而多列子查询则是指查询返回多个列数据的子查询语句案例查询和SMITH的部门和岗位完全相同的所有雇员不含SMITH本人mysql select * from emp where (deptno,job)(select deptno,job from emp where ename‘SMITH’) and ename‘SMITH’; mysql select * from emp where (deptno,job)in(select deptno,job from emp where ename‘SMITH’) and ename‘SMITH’; ±-------±------±------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±------±-----±--------------------±--------±-----±------- | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | ±-------±------±------±-----±--------------------±--------±-----±------- 1 row in set (0.00 sec) ### 4.4在from子句中使用子查询子查询语句出现在from子句中。这里要用到数据查询的技巧把一个子查询当做一个临时表使用。案例显示每个高于自己部门平均工资的员工的姓名、部门、工资、平均工资答案 select t1.ename,t1.deptno,t1.sal,t2.myavg from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptnot2.deptno and t1.ssal t2.myavg; 步骤 // 1.根据部门号分组得到每组的平均工资 mysql select avg(sal) from emp group by deptno; ±------------ | avg(sal) | ±------------ | 2916.666667 | | 2175.000000 | | 1566.666667 | ±------------ 3 rows in set (0.00 sec) // 2.根据部门号分组得到每组的平均工资和部门号 mysql select deptno,avg(sal) from emp group by deptno; ±-------±------------ | deptno | avg(sal) | ±-------±------------ | 10 | 2916.666667 | | 20 | 2175.000000 | | 30 | 1566.666667 | ±-------±------------ 3 rows in set (0.00 sec) // 3.将上面得到的结果与emp表做笛卡尔积 mysql select * from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptnot2.deptno; ±-------±-------±----------±-----±--------------------±--------±--------±-------±-------±------------ | empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | myavg | ±-------±-------±----------±-----±--------------------±--------±--------±-------±-------±------------ | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | 2175.000000 | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 1566.666667 | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | 1566.666667 | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 2175.000000 | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | 1566.666667 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 1566.666667 | | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | 2916.666667 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 2916.666667 | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | 1566.666667 | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | 2175.000000 | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | 1566.666667 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | 2916.666667 | ±-------±-------±----------±-----±--------------------±--------±--------±-------±-------±------------ 14 rows in set (0.00 sec) // 5.增加筛选条件 :工资大于平均工资 mysql select * from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptnot2.deptno and t1.sal t2.myavg; ±-------±------±----------±-----±--------------------±--------±-------±-------±-------±------------ | empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | myavg | ±-------±------±----------±-----±--------------------±--------±-------±-------±-------±------------ | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | 1566.666667 | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | 2175.000000 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 1566.666667 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 2916.666667 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 2175.000000 | ±-------±------±----------±-----±--------------------±--------±-------±-------±-------±------------ 6 rows in set (0.00 sec) // 5.根据题目要求得到结果 mysql select t1.ename,t1.deptno,t1.sal,t2.myavg from emp t1,(select deptno,avg(sal) myavg from emp group by deptno) t2 where t1.deptnot2.deptno and t1.ssal t2.myavg; ±------±-------±--------±------------ | ename | deptno | sal | myavg | ±------±-------±--------±------------ | ALLEN | 30 | 1600.00 | 1566.666667 | | JONES | 20 | 2975.00 | 2175.000000 | | BLAKE | 30 | 2850.00 | 1566.666667 | | SCOTT | 20 | 3000.00 | 2175.000000 | | KING | 10 | 5000.00 | 2916.666667 | | FORD | 20 | 3000.00 | 2175.000000 | ±------±-------±--------±------------ 6 rows in set (0.00 sec)查找每个部门工资最高的人的姓名、工资、部门、最高工资答案 select t1.ename,t1.sal,t1.deptno,t2.mymax from emp t1,(select deptno, max(sal) mymax from emp group by deptno) t2 where t1.deptnot2.deptno and t1…salt2.mymax; 步骤 // 1.得到分组之后的部门号和最高工资 mysql select deptno, max(sal) from emp group by deptno; ±-------±--------- | deptno | max(sal) | ±-------±--------- | 10 | 5000.00 | | 20 | 3000.00 | | 30 | 2850.00 | ±-------±--------- 3 rows in set (0.01 sec) // 2.与emp表进行笛卡尔积并进行t1.salt2.mymax的筛选(工资等于最高工资) mysql select * from emp t1,(select deptno, max(sal) mymax from emp group by deptno) t2 where t1.deptnot2.deptno and t1.salt2.mymax; ±-------±------±----------±-----±--------------------±--------±-----±-------±-------±-------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | mymax | ±-------±------±----------±-----±--------------------±--------±-----±-------±-------±-------- | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | 2850.00 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | 5000.00 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | 3000.00 | ±-------±------±----------±-----±--------------------±--------±-----±-------±-------±-------- 4 rows in set (0.00 sec) // 3.根据题目要求选择需要筛选的内容 mysql select t1.ename,t1.sal,t1.deptno,t2.mymax from emp t1,(select deptno, max(sal) mymax from emp group by deptno) t2 where t1.deptnot2.deptno and t1…salt2.mymax; ±------±--------±-------±-------- | ename | sal | deptno | mymax | ±------±--------±-------±-------- | BLAKE | 2850.00 | 30 | 2850.00 | | SCOTT | 3000.00 | 20 | 3000.00 | | KING | 5000.00 | 10 | 5000.00 | | FORD | 3000.00 | 20 | 3000.00 | ±------±--------±-------±-------- 4 rows in set (0.00 sec显示每个部门的信息部门名编号地址和人员数量答案 select t1.deptno,t1.dname,t1.loc,t2.num from dept t1,(select deptno,count() num from emp group by deptno) t2 where t1.deptnot2.deptno; 步骤 // 1.分组得到每一组的人数 mysql select deptno,count() num from emp group by deptno; ±-------±---- | deptno | num | ±-------±---- | 10 | 3 | | 20 | 5 | | 30 | 6 | ±-------±---- 3 rows in set (0.00 sec) // 2.和部门表进行笛卡尔积然后进行条件筛选 mysql select * from dept t1,(select deptno,count() num from emp group by deptno) t2 where t1.deptnot2.deptno; ±-------±-----------±---------±-------±---- | deptno | dname | loc | deptno | num | ±-------±-----------±---------±-------±---- | 10 | ACCOUNTING | NEW YORK | 10 | 3 | | 20 | RESEARCH | DALLAS | 20 | 5 | | 30 | SALES | CHICAGO | 30 | 6 | ±-------±-----------±---------±-------±---- 3 rows in set (0.01 sec) mysql select t1.deptno,t1.dname,t1.loc,t2.num from dept t1,(select deptno,count() num from emp group by deptno) t2 where t1.deptnot2.deptno; ±-------±-----------±---------±---- | deptno | dname | loc | num | ±-------±-----------±---------±---- | 10 | ACCOUNTING | NEW YORK | 3 | | 20 | RESEARCH | DALLAS | 5 | | 30 | SALES | CHICAGO | 6 | ±-------±-----------±---------±---- 3 rows in set (0.00 sec) 暴力解法 mysql select dept.dname,dept.deptno,dept.loc,count() from emp,dept where emp.deptnodept.deptno group by dept.deptno,dept.dname,dept.loc; ±-----------±-------±---------±--------- | dname | deptno | loc | count() | ±-----------±-------±---------±--------- | ACCOUNTING | 10 | NEW YORK | 3 | | RESEARCH | 20 | DALLAS | 5 | | SALES | 30 | CHICAGO | 6 | ±-----------±-------±---------±--------- 3 rows in set (0.01 sec) 总结解决多表问题的本质想办法将多表转化为单表所以mysql中所有select的问题全部都可以转化成单表问题### 4.5合并查询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 unionunion all#### 4.5.1 union该操作符用于取得两个结果集的并集。当使用该操作符时会自动去掉结果集中的重复行。案例将工资大于2500或职位是MANAGER的人找出// 1.查出工资大于2500的 mysql select * from emp where sal2500; ±-------±------±----------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±----------±-----±--------------------±--------±-----±------- | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | ±-------±------±----------±-----±--------------------±--------±-----±------- 5 rows in set (0.00 sec) // 2.查出jobMANAGER的 mysql select * from emp where job‘MANAGER’; ±-------±------±--------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±--------±-----±--------------------±--------±-----±------- | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | ±-------±------±--------±-----±--------------------±--------±-----±------- 3 rows in set (0.00 sec) // 3.进行合并 mysql select * from emp where sal2500 union select * from emp where job‘MANAGER’; ±------±------±----------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±------±------±----------±-----±--------------------±--------±-----±------- | 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 7782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | ±------±------±----------±-----±--------------------±--------±-----±------- 6 rows in set (0.00 sec) #### 4.5.2 union all操作符用于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。案例将工资大于25000或职位是MANAGER的人找出来// 1.查出工资大于2500的 mysql select * from emp where sal2500; ±-------±------±----------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±----------±-----±--------------------±--------±-----±------- | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | ±-------±------±----------±-----±--------------------±--------±-----±------- 5 rows in set (0.00 sec) // 2.查出jobMANAGER的 mysql select * from emp where job‘MANAGER’; ±-------±------±--------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±-------±------±--------±-----±--------------------±--------±-----±------- | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | ±-------±------±--------±-----±--------------------±--------±-----±------- 3 rows in set (0.01 sec) // 3.进行合并 mysql select * from emp where sal2500 union all select * from emp where job‘MANAGER’; ±------±------±----------±-----±--------------------±--------±-----±------- | empno | ename | job | mgr | hiredate | sal | comm | deptno | ±------±------±----------±-----±--------------------±--------±-----±------- | 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 7788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | | 7839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | | 7902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | | 7566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | | 7698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | | 7782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | ±------±------±----------±-----±--------------------±--------±-----±------- 8 rows in set (0.00 sec)