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平昌移动网站建设,怎么做电影流量网站吗,临沂建网站公司,域名抢注文章目录 前言一、基本查询回顾二、 多表查询解决多表查询的思路 三、自连接四、子查询1. 单行子查询2. 多行子查询3. 多列子查询4. 在from子句中使用子查询5. 合并查询5.1 union5.2 unoin all 总结 前言 前面的学习中,对于mysql表的查询都是对一张表进行查询,在实际开发中这远… 文章目录 前言一、基本查询回顾二、 多表查询解决多表查询的思路 三、自连接四、子查询1. 单行子查询2. 多行子查询3. 多列子查询4. 在from子句中使用子查询5. 合并查询5.1 union5.2 unoin all 总结 前言 前面的学习中,对于mysql表的查询都是对一张表进行查询,在实际开发中这远远是不够的!!! 下面是关于复合查询的讲解! 正文开始!!! 一、基本查询回顾 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

mysql select * from emp where (sal500 or jobMANAGER) 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 jobMANAGER) 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

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 | CLARK | 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 |

14 rows in set (0.00 sec) 使用年薪进行降序排序

mysql select ename,(sal*12comm) 年薪 from emp order by 年薪 desc;

| ename | 年薪 |

| ALLEN | 19500.00 | | TURNER | 18000.00 | | MARTIN | 16400.00 | | WARD | 15500.00 | | SMITH | NULL | | JONES | NULL | | BLAKE | NULL | | CLARK | NULL | | SCOTT | NULL | | KING | NULL | | ADAMS | NULL | | JAMES | NULL | | FORD | NULL |

| MILLER | NULL |

14 rows in set (0.00 sec)mysql select ename,(sal*12ifnull(comm,0.0)) 年薪 from emp order by 年薪 desc;

| ename | 年薪 |

| KING | 60000.00 | | SCOTT | 36000.00 | | FORD | 36000.00 | | JONES | 35700.00 | | BLAKE | 34200.00 | | CLARK | 29400.00 | | ALLEN | 19500.00 | | TURNER | 18000.00 | | MARTIN | 16400.00 | | MILLER | 15600.00 | | WARD | 15500.00 | | ADAMS | 13200.00 | | JAMES | 11400.00 |

| SMITH | 9600.00 |

14 rows in set (0.00 sec)显示工资最高的员工的名字和工作岗位

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

mysql select ename,job from emp where sal(select avg(sal) from emp);

| ename | job |

| JONES | MANAGER | | BLAKE | MANAGER | | CLARK | MANAGER | | SCOTT | ANALYST | | KING | PRESIDENT |

| FORD | ANALYST |

6 rows in set (0.00 sec)显示每个部门的平均工资和最高工资

mysql select deptno,avg(sal),max(sal) from emp group by deptno;

| deptno | avg(sal) | max(sal) |

| 10 | 2916.666667 | 5000.00 | | 20 | 2175.000000 | 3000.00 |

| 30 | 1566.666667 | 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

mysql select deptno,avg(sal) myavg from emp group by deptno having myavg2000;

| deptno | myavg |

| 30 | 1566.666667 |

1 row in set (0.00 sec) 显示每种岗位的雇员总数平均工资

mysql select job,count(*),avg(sal) myavg from emp group by job;

| job | count(*) | myavg |

| 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)二、 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.现在我们继续使用之前的三张表(emp,dept,salgarde)来演示如何进行多表查询.

mysql select * from emp,dept;

| empno | ename | job | mgr | hiredate | sal | comm | deptno | deptno | dname | loc |

| 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK | | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 30 | SALES | CHICAGO | | 007369 | SMITH | CLERK | 7902 | 1980-12-17 00:00:00 | 800.00 | NULL | 20 | 40 | OPERATIONS | BOSTON | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 10 | ACCOUNTING | NEW YORK | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 20 | RESEARCH | DALLAS | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 30 | SALES | CHICAGO | | 007499 | ALLEN | SALESMAN | 7698 | 1981-02-20 00:00:00 | 1600.00 | 300.00 | 30 | 40 | OPERATIONS | BOSTON | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 10 | ACCOUNTING | NEW YORK | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 20 | RESEARCH | DALLAS | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 30 | SALES | CHICAGO | | 007521 | WARD | SALESMAN | 7698 | 1981-02-22 00:00:00 | 1250.00 | 500.00 | 30 | 40 | OPERATIONS | BOSTON | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 30 | SALES | CHICAGO | | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL | 20 | 40 | OPERATIONS | BOSTON | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 10 | ACCOUNTING | NEW YORK | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 20 | RESEARCH | DALLAS | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 30 | SALES | CHICAGO | | 007654 | MARTIN | SALESMAN | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 | 30 | 40 | OPERATIONS | BOSTON | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 10 | ACCOUNTING | NEW YORK | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 20 | RESEARCH | DALLAS | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 30 | SALES | CHICAGO | | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL | 30 | 40 | OPERATIONS | BOSTON | | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK | | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 20 | RESEARCH | DALLAS | | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 30 | SALES | CHICAGO | | 007782 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | 40 | OPERATIONS | BOSTON | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 30 | SALES | CHICAGO | | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL | 20 | 40 | OPERATIONS | BOSTON | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 20 | RESEARCH | DALLAS | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 30 | SALES | CHICAGO | | 007839 | KING | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL | 10 | 40 | OPERATIONS | BOSTON | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 10 | ACCOUNTING | NEW YORK | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 20 | RESEARCH | DALLAS | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 30 | SALES | CHICAGO | | 007844 | TURNER | SALESMAN | 7698 | 1981-09-08 00:00:00 | 1500.00 | 0.00 | 30 | 40 | OPERATIONS | BOSTON | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 30 | SALES | CHICAGO | | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL | 20 | 40 | OPERATIONS | BOSTON | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 10 | ACCOUNTING | NEW YORK | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 20 | RESEARCH | DALLAS | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 30 | SALES | CHICAGO | | 007900 | JAMES | CLERK | 7698 | 1981-12-03 00:00:00 | 950.00 | NULL | 30 | 40 | OPERATIONS | BOSTON | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 10 | ACCOUNTING | NEW YORK | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 20 | RESEARCH | DALLAS | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 30 | SALES | CHICAGO | | 007902 | FORD | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL | 20 | 40 | OPERATIONS | BOSTON | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 10 | ACCOUNTING | NEW YORK | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 20 | RESEARCH | DALLAS | | 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 30 | SALES | CHICAGO |

| 007934 | MILLER | CLERK | 7782 | 1982-01-23 00:00:00 | 1300.00 | NULL | 10 | 40 | OPERATIONS | BOSTON |

56 rows in set (0.00 sec)本质:数据的穷举!!! 我们依旧认为这是一张表! 解决多表查询的思路 先读题,确定都和哪些表有关.无脑组合形成一张表 – 多张表转化称为一张表将多表查询,看做称为一张表的查询. 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

mysql select emp.ename,emp.sal,dept.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 | | CLARK | 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)# 如果属性字段唯一的话,可以省略select后面的.操作

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 | | CLARK | 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 emp.deptno,dept.dname,emp.ename,emp.sal from emp,dept where emp.deptnodept.deptno and emp.deptno10;

| deptno | dname | ename | sal |

| 10 | ACCOUNTING | CLARK | 2450.00 | | 10 | ACCOUNTING | KING | 5000.00 |

| 10 | 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 emp.ename,emp.sal,salgrade.grade from emp,salgrade where emp.sal between salgrade.losal and salgrade.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 | | CLARK | 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)三、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

mysql select empno,ename from emp where empno(select mgr from emp where enameFORD);

| empno | ename |

| 007566 | JONES |

1 row in set (0.00 sec)使用多表查询(自查询) – 使用到表的别名

–from emp t1, emp t2给自己的表起别名因为要先做笛卡尔积所以别名可以先识别mysql select t2.empno,t2.ename from emp t1,emp t2 where t1.enameFORD and t1.mgrt2.empno;

| empno | ename |

| 007566 | JONES |

1 row in set (0.00 sec) 四、子查询 子查询是指嵌入在其他sql语句中的select语句,也叫作嵌套查询.

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 mysql select * from emp WHERE deptno (select deptno from emp where enameSMITH); ————————————————————————– | 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)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 mysql select ename,job,sal,deptno from emp where (job in (select job from emp where deptno10)) and deptno!10; ——————————— | 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. 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 (0.00 sec)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;显示比部门30的任意员工的工资搞的员工的姓名,工资和部门号(包含自己部门的员工) mysql select ename,sal,deptno from emp where sal(select min(sal) from emp where deptno30); ————————- | ename | sal | deptno | ————————- | ALLEN | 1600.00 | 30 | | WARD | 1250.00 | 30 | | JONES | 2975.00 | 20 | | MARTIN | 1250.00 | 30 | | BLAKE | 2850.00 | 30 | | CLARK | 2450.00 | 10 | | SCOTT | 3000.00 | 20 | | KING | 5000.00 | 10 | | TURNER | 1500.00 | 30 | | ADAMS | 1100.00 | 20 | | FORD | 3000.00 | 20 | | MILLER | 1300.00 | 10 | ————————- 12 rows in set (0.00 sec)mysql select ename,sal,deptno from emp where salany(select sal from emp where deptno30); ————————- | ename | sal | deptno | ————————- | ALLEN | 1600.00 | 30 | | WARD | 1250.00 | 30 | | JONES | 2975.00 | 20 | | MARTIN | 1250.00 | 30 | | BLAKE | 2850.00 | 30 | | CLARK | 2450.00 | 10 | | SCOTT | 3000.00 | 20 | | KING | 5000.00 | 10 | | TURNER | 1500.00 | 30 | | ADAMS | 1100.00 | 20 | | FORD | 3000.00 | 20 | | MILLER | 1300.00 | 10 | ————————- 12 rows in set (0.00 sec)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的多行数据,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 enameSMITH) 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. 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. //获取各个部门的平均工资将其看作临时表 mysql select emp.ename,emp.deptno,emp.sal,tmp.myavg from emp,(select deptno,avg(sal) myavg from emp group by deptno) tmp where emp.deptnotmp.deptno and emp.saltmp.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 mysql select emp.ename,emp.sal,emp.deptno,tmp.mymax from emp,(select deptno,max(sal) mymax from emp group by deptno) tmp where emp.deptnotmp.deptno and emp.saltmp.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 方法1使用多表
   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 (0.00 sec)使用子查询 – 1. 对EMP表进行人员统计 – 2. 将上面的表看作临时表mysql select dept.dname,dept.deptno,dept.loc,tmp.总人数 from dept,(select deptno,count(*) 总人数 from emp group by deptno) tmp where dept.deptnotmp.deptno; —————————————– | dname | deptno | loc | 总人数 | —————————————– | ACCOUNTING | 10 | NEW YORK | 3 | | RESEARCH | 20 | DALLAS | 5 | | SALES | 30 | CHICAGO | 6 | —————————————– 3 rows in set (0.00 sec)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union,union all 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或者职位是’MANAGER’的人找出来 mysql select * from emp where sal2500 union select * from emp where jobMANAGER; ————————————————————————— | 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 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | ————————————————————————— 6 rows in set (0.00 sec) –去掉了重复记录 5.2 unoin all 该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2500或者职位是’MANAGER’的人找出来 mysql select * from emp where sal2500 union all select * from emp where jobMANAGER; ————————————————————————— | 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 | CLARK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL | 10 | ————————————————————————— 8 rows in set (0.00 sec) 总结 (本章完!!!)