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标题: 【MySQL】MySQL复合查询--多表查询/自连接/子查询 [打印本页]

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作者: 杀鸡焉用牛刀    时间: 2024-7-17 08:29
标题: 【MySQL】MySQL复合查询--多表查询/自连接/子查询
1.根本查询回首

表的内容如下：

1. mysql> select * from emp;
2. +--------+--------+-----------+------+---------------------+---------+---------+--------+
3. | empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
4. +--------+--------+-----------+------+---------------------+---------+---------+--------+
5. | 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
6. | 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
7. | 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
8. | 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
9. | 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
10. | 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
11. | 007782 | CLARK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
12. | 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
13. | 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
14. | 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
15. | 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
16. | 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
17. | 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
18. | 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
19. +--------+--------+-----------+------+---------------------+---------+---------+--------+
20. 14 rows in set (0.00 sec)
22. mysql> select * from dept;
23. +--------+------------+----------+
24. | deptno | dname      | loc      |
25. +--------+------------+----------+
26. |     10 | ACCOUNTING | NEW YORK |
27. |     20 | RESEARCH   | DALLAS   |
28. |     30 | SALES      | CHICAGO  |
29. |     40 | OPERATIONS | BOSTON   |
30. +--------+------------+----------+
31. 4 rows in set (0.00 sec)
33. mysql> select * from salgrade;
34. +-------+-------+-------+
35. | grade | losal | hisal |
36. +-------+-------+-------+
37. |     1 |   700 |  1200 |
38. |     2 |  1201 |  1400 |
39. |     3 |  1401 |  2000 |
40. |     4 |  2001 |  3000 |
41. |     5 |  3001 |  9999 |
42. +-------+-------+-------+
43. 5 rows in set (0.00 sec)

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• 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
  

1. // 使用模糊查询
2. select * from emp where (sal>500 or job='MANAGER') and ename like 'J%';
3. // 使用函数
4. select * from emp where (sal>500 or job='MANAGER') and substring(ename,1,1)='J';
5. mysql> select * from emp where (sal>500 or job='MANAGER') and ename like 'J%';
6. +--------+-------+---------+------+---------------------+---------+------+--------+
7. | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
8. +--------+-------+---------+------+---------------------+---------+------+--------+
9. | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
10. | 007900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
11. +--------+-------+---------+------+---------------------+---------+------+--------+
12. 2 rows in set (0.00 sec)
14. mysql> select * from emp where (sal>500 or job='MANAGER') and substring(ename,1,1)='J';
15. +--------+-------+---------+------+---------------------+---------+------+--------+
16. | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
17. +--------+-------+---------+------+---------------------+---------+------+--------+
18. | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
19. | 007900 | JAMES | CLERK   | 7698 | 1981-12-03 00:00:00 |  950.00 | NULL |     30 |
20. +--------+-------+---------+------+---------------------+---------+------+--------+
21. 2 rows in set (0.00 sec)

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• 按照部门号升序而雇员的工资降序排序
  

1. select * from emp order by deptno asc, sal desc;
2. mysql> select * from emp order by deptno asc,sal desc;
3. +--------+--------+-----------+------+---------------------+---------+---------+--------+
4. | empno  | ename  | job       | mgr  | hiredate            | sal     | comm    | deptno |
5. +--------+--------+-----------+------+---------------------+---------+---------+--------+
6. | 007839 | KING   | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 |    NULL |     10 |
7. | 007782 | CLACK  | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 |    NULL |     10 |
8. | 007934 | MILLER | CLERK     | 7782 | 1982-01-23 00:00:00 | 1300.00 |    NULL |     10 |
9. | 007788 | SCOTT  | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 |    NULL |     20 |
10. | 007902 | FORD   | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 |    NULL |     20 |
11. | 007566 | JONES  | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 |    NULL |     20 |
12. | 007876 | ADAMS  | CLERK     | 7788 | 1987-05-23 00:00:00 | 1100.00 |    NULL |     20 |
13. | 007369 | SMITH  | CLERK     | 7902 | 1980-12-17 00:00:00 |  800.00 |    NULL |     20 |
14. | 007698 | BLAKE  | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 |    NULL |     30 |
15. | 007499 | ALLEN  | SALESMAN  | 7698 | 1981-02-20 00:00:00 | 1600.00 |  300.00 |     30 |
16. | 007844 | TURNER | SALESMAN  | 7698 | 1981-09-08 00:00:00 | 1500.00 |    0.00 |     30 |
17. | 007521 | WARD   | SALESMAN  | 7698 | 1981-02-22 00:00:00 | 1250.00 |  500.00 |     30 |
18. | 007654 | MARTIN | SALESMAN  | 7698 | 1981-09-28 00:00:00 | 1250.00 | 1400.00 |     30 |
19. | 007900 | JAMES  | CLERK     | 7698 | 1981-12-03 00:00:00 |  950.00 |    NULL |     30 |
20. +--------+--------+-----------+------+---------------------+---------+---------+--------+

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• 利用年薪进行降序排序
  年薪等于工资*12+奖金
  必要对奖金进行判定，如果奖金为null，则奖金为0
  

1. select ename, sal*12+ifnull(comm,0) as '年薪' from emp order by 年薪 desc;
3. mysql> select ename,sal*12+ifnull(comm,0) as '年薪' from emp  order by 年薪 desc;
4. +--------+----------+
5. | ename  | 年薪     |
6. +--------+----------+
7. | SMITH  |  9600.00 |
8. | ALLEN  | 19500.00 |
9. | WARD   | 15500.00 |
10. | JONES  | 35700.00 |
11. | MARTIN | 16400.00 |
12. | BLAKE  | 34200.00 |
13. | TEST   | 29400.00 |
14. | SCOTT  | 36000.00 |
15. | KING   | 60000.00 |
16. | TURNER | 18000.00 |
17. | ADAMS  | 13200.00 |
18. | JAMES  | 11400.00 |
19. | FORD   | 36000.00 |
20. | MILLER | 15600.00 |
21. +--------+----------+
22. 14 rows in set (0.00 sec)

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• 显示工资最高的员工的名字和工作岗位
  这里利用分组查询即可，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
  

1. select ename,job from emp where sal = (select max(sal) from emp);
3. mysql> select ename,job from emp where sal = (select max(sal) from emp);
4. +-------+-----------+
5. | ename | job       |
6. +-------+-----------+
7. | KING  | PRESIDENT |
8. +-------+-----------+
9. 1 row in set (0.00 sec)

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• 显示工资高于平均工资的员工信息
  这里利用分组查询即可
  

1. select ename,sal from emp where sal > (select avg(sal) from emp);
3. mysql> select ename,sal from emp where sal > (select avg(sal) from emp);
4. +-------+---------+
5. | ename | sal     |
6. +-------+---------+
7. | JONES | 2975.00 |
8. | BLAKE | 2850.00 |
9. | TEST  | 2450.00 |
10. | SCOTT | 3000.00 |
11. | KING  | 5000.00 |
12. | FORD  | 3000.00 |
13. +-------+---------+
14. 6 rows in set (0.00 sec)

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• 显示每个部门的平均工资和最高工资
  

1. select deptno,avg(sal),max(sal) from emp group by deptno;
3. mysql> select deptno,avg(sal),max(sal) from emp group by deptno;
4. +--------+-------------+----------+
5. | deptno | avg(sal)    | max(sal) |
6. +--------+-------------+----------+
7. |     10 | 2425.000000 |  5000.00 |
8. |     20 | 2175.000000 |  3000.00 |
9. |     30 | 1690.000000 |  2850.00 |
10. +--------+-------------+----------+
11. 3 rows in set (0.00 sec)

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• 显示平均工资低于2000的部门号和它的平均工资
  

1. select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000;
3. mysql> select deptno,avg(sal) as avg_sal from emp group by deptno having avg_sal < 2000;
4. +--------+-------------+
5. | deptno | avg_sal     |
6. +--------+-------------+
7. |     30 | 1690.000000 |
8. +--------+-------------+
9. 1 row in set (0.00 sec)

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• 显示每种岗位的雇员总数，平均工资
  

1. select job,count(*), avg(sal) from emp group by job;
3. mysql> select job,count(*), avg(sal) from emp group by job;
4. +-----------+----------+-------------+
5. | job       | count(*) | avg(sal)    |
6. +-----------+----------+-------------+
7. | ANALYST   |        2 | 3000.000000 |
8. | CLERK     |        4 | 1037.500000 |
9. | MANAGER   |        3 | 2758.333333 |
10. | PRESIDENT |        1 | 5000.000000 |
11. | SALESMAN  |        4 | 1400.000000 |
12. +-----------+----------+-------------+
13. 5 rows in set (0.00 sec)

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2.多表查询

现实开发中每每数据来自不同的表，所以必要多表查询。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、雇员工资以及地点部门的名字由于上面的数据来自emp和dept表，因此要团结查询

其实我们只要emp表中的deptno = dept表中的deptno字段的纪录

1. select ename,sal,dname from emp,dept where emp.deptno=dept.deptno;
2. mysql> select ename,sal,dname from emp,dept where emp.deptno=dept.deptno;
3. +--------+---------+------------+
4. | ename  | sal     | dname      |
5. +--------+---------+------------+
6. | SMITH  |  800.00 | RESEARCH   |
7. | ALLEN  | 1600.00 | SALES      |
8. | WARD   | 1250.00 | SALES      |
9. | JONES  | 2975.00 | RESEARCH   |
10. | MARTIN | 1250.00 | SALES      |
11. | BLAKE  | 2850.00 | SALES      |
12. | CLACK  | 2450.00 | ACCOUNTING |
13. | SCOTT  | 3000.00 | RESEARCH   |
14. | KING   | 5000.00 | ACCOUNTING |
15. | TURNER | 1500.00 | SALES      |
16. | ADAMS  | 1100.00 | RESEARCH   |
17. | JAMES  |  950.00 | SALES      |
18. | FORD   | 3000.00 | RESEARCH   |
19. | MILLER | 1300.00 | ACCOUNTING |
20. +--------+---------+------------+
21. 14 rows in set (0.00 sec)

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• 显示部门号为10的部门名，员工名和工资
  

1. mysql> select dname,ename,sal from emp,dept where emp.deptno=dept.deptno and dept.deptno=10;
2. +------------+--------+---------+
3. | dname      | ename  | sal     |
4. +------------+--------+---------+
5. | ACCOUNTING | CLACK  | 2450.00 |
6. | ACCOUNTING | KING   | 5000.00 |
7. | ACCOUNTING | MILLER | 1300.00 |
8. +------------+--------+---------+
9. 3 rows in set (0.00 sec)

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• 显示各个员工的姓名，工资，及工资级别
  

1. mysql> select ename,sal,grade from emp,salgrade where sal between losal and hisal;
2. mysql> select ename,sal,grade from emp,salgrade where sal between losal and hisal;
3. +--------+---------+-------+
4. | ename  | sal     | grade |
5. +--------+---------+-------+
6. | SMITH  |  800.00 |     1 |
7. | ALLEN  | 1600.00 |     3 |
8. | WARD   | 1250.00 |     2 |
9. | JONES  | 2975.00 |     4 |
10. | MARTIN | 1250.00 |     2 |
11. | BLAKE  | 2850.00 |     4 |
12. | CLACK  | 2450.00 |     4 |
13. | SCOTT  | 3000.00 |     4 |
14. | KING   | 5000.00 |     5 |
15. | TURNER | 1500.00 |     3 |
16. | ADAMS  | 1100.00 |     1 |
17. | JAMES  |  950.00 |     1 |
18. | FORD   | 3000.00 |     4 |
19. | MILLER | 1300.00 |     2 |
20. +--------+---------+-------+
21. 14 rows in set (0.00 sec)

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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）
利用的子查询

1. select ename,empno from emp where empno=(select mgr from emp where ename='FORD');

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利用多表查询（自查询）

1. select e2.ename,e2.empno from emp e1,emp e2 where e1.ename='FORD' and e1.mgr=e2.empno;
2. mysql> select e1.ename,e2.empno from emp e1,emp e2 where e1.ename='FORD' and e1.mgr=e2.empno;
3. +-------+--------+
4. | ename | empno  |
5. +-------+--------+
6. | FORD  | 007566 |
7. +-------+--------+
8. 1 row in set (0.00 sec)

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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

返回一行纪录的子查询

• 显示SMITH同一部门的员工
  

1. select * from emp where deptno=(select deptno from emp where ename='SMITH');
2. mysql> select * from emp where deptno=(select deptno from emp where ename='SMITH');
3. +--------+-------+---------+------+---------------------+---------+------+--------+
4. | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
5. +--------+-------+---------+------+---------------------+---------+------+--------+
6. | 007369 | SMITH | CLERK   | 7902 | 1980-12-17 00:00:00 |  800.00 | NULL |     20 |
7. | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
8. | 007788 | SCOTT | ANALYST | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
9. | 007876 | ADAMS | CLERK   | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
10. | 007902 | FORD  | ANALYST | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
11. +--------+-------+---------+------+---------------------+---------+------+--------+
12. 5 rows in set (0.00 sec)

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4.2多行子查询

返回多行纪录的子查询

• 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
  
  1. select ename,job,sal,deptno from emp where job in(select job from emp where deptno=10) and deptno<>10;
  2. mysql> select ename,job,sal,deptno from emp where job in(select job from emp where deptno=10) and deptno<>10;
  3. +-------+---------+---------+--------+
  4. | ename | job     | sal     | deptno |
  5. +-------+---------+---------+--------+
  6. | JONES | MANAGER | 2975.00 |     20 |
  7. | BLAKE | MANAGER | 2850.00 |     30 |
  8. | SMITH | CLERK   |  800.00 |     20 |
  9. | ADAMS | CLERK   | 1100.00 |     20 |
  10. | JAMES | CLERK   |  950.00 |     30 |
  11. +-------+---------+---------+--------+
  12. 5 rows in set (0.00 sec)

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• 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
  
  1. // 使用聚合函数
  2. select ename,sal,deptno from emp where sal>(select max(sal) from emp where deptno=30);
  3. mysql> select ename,sal,deptno from emp where sal>(select max(sal) from emp where deptno=30);
  4. +-------+---------+--------+
  5. | ename | sal     | deptno |
  6. +-------+---------+--------+
  7. | JONES | 2975.00 |     20 |
  8. | SCOTT | 3000.00 |     20 |
  9. | KING  | 5000.00 |     10 |
  10. | FORD  | 3000.00 |     20 |
  11. +-------+---------+--------+
  12. 4 rows in set (0.01 sec)
  14. // 使用all关键子
  15. select ename,sal,deptno from emp where sal>all(select sal from emp where deptno=30);
  16. mysql> select ename,sal,deptno from emp where sal>all(select sal from emp where deptno=30);
  17. +-------+---------+--------+
  18. | ename | sal     | deptno |
  19. +-------+---------+--------+
  20. | JONES | 2975.00 |     20 |
  21. | SCOTT | 3000.00 |     20 |
  22. | KING  | 5000.00 |     10 |
  23. | FORD  | 3000.00 |     20 |
  24. +-------+---------+--------+
  25. 4 rows in set (0.00 sec)

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• 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）
  

1. // 使用聚合函数
2. mysql> select ename,sal,deptno from emp where sal > (select min(sal) from emp where deptno=30) and deptno<>30;
3. +--------+---------+--------+
4. | ename  | sal     | deptno |
5. +--------+---------+--------+
6. | JONES  | 2975.00 |     20 |
7. | CLACK  | 2450.00 |     10 |
8. | SCOTT  | 3000.00 |     20 |
9. | KING   | 5000.00 |     10 |
10. | ADAMS  | 1100.00 |     20 |
11. | FORD   | 3000.00 |     20 |
12. | MILLER | 1300.00 |     10 |
13. +--------+---------+--------+
14. 7 rows in set (0.00 sec)
15. // 使用any关键字
16. mysql> select ename,sal,deptno from emp where sal > any(select sal from emp where deptno=30) and deptno<>30;
17. +--------+---------+--------+
18. | ename  | sal     | deptno |
19. +--------+---------+--------+
20. | JONES  | 2975.00 |     20 |
21. | CLACK  | 2450.00 |     10 |
22. | SCOTT  | 3000.00 |     20 |
23. | KING   | 5000.00 |     10 |
24. | ADAMS  | 1100.00 |     20 |
25. | FORD   | 3000.00 |     20 |
26. | MILLER | 1300.00 |     10 |
27. +--------+---------+--------+
28. 7 rows in set (0.00 sec)

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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

1. mysql> select * from emp where (deptno,job)=(select deptno,job from emp where ename='SMITH') and ename<>'SMITH';
2. mysql> select * from emp where (deptno,job)in(select deptno,job from emp where ename='SMITH') and ename<>'SMITH';
3. +--------+-------+-------+------+---------------------+---------+------+--------+
4. | empno  | ename | job   | mgr  | hiredate            | sal     | comm | deptno |
5. +--------+-------+-------+------+---------------------+---------+------+--------+
6. | 007876 | ADAMS | CLERK | 7788 | 1987-05-23 00:00:00 | 1100.00 | NULL |     20 |
7. +--------+-------+-------+------+---------------------+---------+------+--------+
8. 1 row in set (0.00 sec)

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4.4在from子句中利用子查询

子查询语句出现在from子句中。这里要用到数据查询的本领，把一个子查询当做一个临时表利用。
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：

1. 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.deptno=t2.deptno and t1.ssal > t2.myavg;

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步骤：

1. // 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.将上面得到的效果与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.deptno=t2.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.增长筛选条件 :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.deptno=t2.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.根据题目要求得到效果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.deptno=t2.deptno and t1.ssal > t2.myavg;
2. +-------+--------+---------+-------------+| 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)

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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：

1. 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.deptno=t2.deptno and t1..sal=t2.mymax;

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步骤：

1. // 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.sal=t2.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.deptno=t2.deptno and t1.sal=t2.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.根据题目要求选择必要筛选的内容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.deptno=t2.deptno and t1..sal=t2.mymax;
2. +-------+---------+--------+---------+| 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

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显示每个部门的信息（部门名，编号，地点）和职员数量
答案：

1. select t1.deptno,t1.dname,t1.loc,t2.num from dept t1,(select deptno,count(*) num from emp group by deptno) t2 where t1.deptno=t2.deptno;

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步骤：

1. // 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.deptno=t2.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.deptno=t2.deptno;
2. +--------+------------+----------+-----+| deptno | dname      | loc      | num |+--------+------------+----------+-----+|     10 | ACCOUNTING | NEW YORK |   3 ||     20 | RESEARCH   | DALLAS   |   5 ||     30 | SALES      | CHICAGO  |   6 |+--------+------------+----------+-----+3 rows in set (0.00 sec)

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暴力解法：

1. mysql> select dept.dname,dept.deptno,dept.loc,count(*) from emp,dept where emp.deptno=dept.deptno group by dept.deptno,dept.dname,dept.loc;
2. +------------+--------+----------+----------+
3. | dname      | deptno | loc      | count(*) |
4. +------------+--------+----------+----------+
5. | ACCOUNTING |     10 | NEW YORK |        3 |
6. | RESEARCH   |     20 | DALLAS   |        5 |
7. | SALES      |     30 | CHICAGO  |        6 |
8. +------------+--------+----------+----------+
9. 3 rows in set (0.01 sec)

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总结：
办理多表问题的本质：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

在现实应用中，为了合并多个select的实行效果，可以利用集合操纵符 union，union all
4.5.1 union

该操纵符用于取得两个效果集的并集。当利用该操纵符时，会自动去掉效果会合的重复行。
案例：将工资大于2500或职位是MANAGER的人找出

1. // 1.查出工资大于2500的
2. mysql> select * from emp where sal>2500;
3. +--------+-------+-----------+------+---------------------+---------+------+--------+
4. | empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
5. +--------+-------+-----------+------+---------------------+---------+------+--------+
6. | 007566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
7. | 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
8. | 007788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
9. | 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
10. | 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
11. +--------+-------+-----------+------+---------------------+---------+------+--------+
12. 5 rows in set (0.00 sec)
14. // 2.查出job=MANAGER的
15. mysql> select * from emp where job='MANAGER';
16. +--------+-------+---------+------+---------------------+---------+------+--------+
17. | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
18. +--------+-------+---------+------+---------------------+---------+------+--------+
19. | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
20. | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
21. | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 |
22. +--------+-------+---------+------+---------------------+---------+------+--------+
23. 3 rows in set (0.00 sec)
25. // 3.进行合并
26. mysql> select * from emp where sal>2500 union select * from emp where job='MANAGER';
27. +-------+-------+-----------+------+---------------------+---------+------+--------+
28. | empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
29. +-------+-------+-----------+------+---------------------+---------+------+--------+
30. |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
31. |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
32. |  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
33. |  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
34. |  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
35. |  7782 | CLACK | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 |
36. +-------+-------+-----------+------+---------------------+---------+------+--------+
37. 6 rows in set (0.00 sec)

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4.5.2 union all

操纵符用于取得两个效果集的并集。当利用该操纵符时，不会去掉效果会合的重复行。
案例：将工资大于25000或职位是MANAGER的人找出来

1. // 1.查出工资大于2500的
2. mysql> select * from emp where sal>2500;
3. +--------+-------+-----------+------+---------------------+---------+------+--------+
4. | empno  | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
5. +--------+-------+-----------+------+---------------------+---------+------+--------+
6. | 007566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
7. | 007698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
8. | 007788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
9. | 007839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
10. | 007902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
11. +--------+-------+-----------+------+---------------------+---------+------+--------+
12. 5 rows in set (0.00 sec)
14. // 2.查出job=MANAGER的
15. mysql> select * from emp where job='MANAGER';
16. +--------+-------+---------+------+---------------------+---------+------+--------+
17. | empno  | ename | job     | mgr  | hiredate            | sal     | comm | deptno |
18. +--------+-------+---------+------+---------------------+---------+------+--------+
19. | 007566 | JONES | MANAGER | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
20. | 007698 | BLAKE | MANAGER | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
21. | 007782 | CLACK | MANAGER | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 |
22. +--------+-------+---------+------+---------------------+---------+------+--------+
23. 3 rows in set (0.01 sec)
25. // 3.进行合并
26. mysql> select * from emp where sal>2500 union all select * from emp where job='MANAGER';
27. +-------+-------+-----------+------+---------------------+---------+------+--------+
28. | empno | ename | job       | mgr  | hiredate            | sal     | comm | deptno |
29. +-------+-------+-----------+------+---------------------+---------+------+--------+
30. |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
31. |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
32. |  7788 | SCOTT | ANALYST   | 7566 | 1987-04-19 00:00:00 | 3000.00 | NULL |     20 |
33. |  7839 | KING  | PRESIDENT | NULL | 1981-11-17 00:00:00 | 5000.00 | NULL |     10 |
34. |  7902 | FORD  | ANALYST   | 7566 | 1981-12-03 00:00:00 | 3000.00 | NULL |     20 |
35. |  7566 | JONES | MANAGER   | 7839 | 1981-04-02 00:00:00 | 2975.00 | NULL |     20 |
36. |  7698 | BLAKE | MANAGER   | 7839 | 1981-05-01 00:00:00 | 2850.00 | NULL |     30 |
37. |  7782 | CLACK | MANAGER   | 7839 | 1981-06-09 00:00:00 | 2450.00 | NULL |     10 |
38. +-------+-------+-----------+------+---------------------+---------+------+--------+
39. 8 rows in set (0.00 sec)

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