mysql 学习笔记多表查询02

把一张表想象成两张表，进行多表查询

举例：

查询所有员工的姓名以及其上级姓名

select s1.stname, s2.stname from staff as s1, staff as s2 where s1.stmgr = s2.stid;

查询员工李岩的上级姓名

select s1.stname, s2.stname from staff as s1, staff as s2 where s1.stmgr = s2.stid and s1.stname='李岩';

以上是利用表的多表查询
还可以使用子查询来实现
比如：

 select staff.stname from staff where stid = (select stmgr from staff where stname='李岩');

复杂一些的表查询举例：
A：
有两张表，分别是员工信息表，与部门信息表，要求查询，各个部门工资最高的员工的信息
员工表：

mysql> select * from employee;
+----+-------+--------+-------+
| id | name  | salary | depid |
+----+-------+--------+-------+
|  1 | Joe   |  70000 |     1 |
|  2 | Tom   |  80000 |     1 |
|  3 | Mary  |  50000 |     2 |
|  4 | Tk    |  10000 |     3 |
|  5 | Inter |  20000 |     3 |
|  6 | Janet | 780000 |     3 |
|  7 | Li    |  75000 |     1 |
|  8 | Wang  |   2000 |     3 |
|  9 | Gao   |   5000 |     2 |
| 10 | ZhaoF |   1000 |     2 |
| 11 | ZhaoX |   2000 |     2 |
| 12 | Mx    |   5000 |     1 |
| 13 | Mi    |   6000 |     1 |
+----+-------+--------+-------+

部门表：

mysql> select * from department;
+----+--------+
| id | name   |
+----+--------+
|  1 | it     |
|  2 | kuaiji |
|  3 | yunwei |
+----+--------+

思考步骤：
1、从employee表里查询出每个部门的最高薪资，作为一张临时表 t

mysql> select depid , max(salary) as maxsalary from employee group by depid;
+-------+-----------+
| depid | maxsalary |
+-------+-----------+
|     1 |     80000 |
|     2 |     50000 |
|     3 |    780000 |
+-------+-----------+

2、将临时表 t 和 employee表进行内连接，并新增一列，显示employee表里每个员工所在部门对应的最高薪资

mysql> select e.id,e.name,e.salary, t.maxsalary,t.depid from (select depid, max(salary) as maxsalary from employee group by depid) as t inner join employee as e on t.depid=e.depid;
+----+-------+--------+-----------+-------+
| id | name  | salary | maxsalary | depid |
+----+-------+--------+-----------+-------+
|  1 | Joe   |  70000 |     80000 |     1 |
|  2 | Tom   |  80000 |     80000 |     1 |
|  3 | Mary  |  50000 |     50000 |     2 |
|  4 | Tk    |  10000 |    780000 |     3 |
|  5 | Inter |  20000 |    780000 |     3 |
|  6 | Janet | 780000 |    780000 |     3 |
|  7 | Li    |  75000 |     80000 |     1 |
|  8 | Wang  |   2000 |    780000 |     3 |
|  9 | Gao   |   5000 |     50000 |     2 |
| 10 | ZhaoF |   1000 |     50000 |     2 |
| 11 | ZhaoX |   2000 |     50000 |     2 |
| 12 | Mx    |   5000 |     80000 |     1 |
| 13 | Mi    |   6000 |     80000 |     1 |
+----+-------+--------+-----------+-------+

3、再用 employee表里每个员工的薪资字段salary 和部门最高薪资字段列maxsalary进行判断，查询出相等数据，此处则查询出了每个部门最高薪资的员工有哪些，作为表 tt

mysql> select e.id,e.name,e.salary, t.maxsalary,t.depid from (select depid, max(salary) as maxsalary from employee group by depid) as t inner join employee as e on t.depid=e.depid where e.salary=t.maxsalary;
+----+-------+--------+-----------+-------+
| id | name  | salary | maxsalary | depid |
+----+-------+--------+-----------+-------+
|  2 | Tom   |  80000 |     80000 |     1 |
|  3 | Mary  |  50000 |     50000 |     2 |
|  6 | Janet | 780000 |    780000 |     3 |
+----+-------+--------+-----------+-------+

4、因为表 tt 没有部门名称，所以我们再将表 tt 和department 表进行内链接，查询部门id相等的数据，从而查询出每个员工所在的部门名称

mysql> select tt.*, d.name as departname from (select e.id,e.name,e.salary, t.maxsalary,t.depid as tepid from (select depid, max(salary) as maxsalary from employee group by depid) as t inner join employee as e on t.depid=e.depid where e.salary=t.maxsalary) as tt inner join department as d on tt.tepid=d.id order by tt.tepid, tt.id;
+----+-------+--------+-----------+-------+------------+
| id | name  | salary | maxsalary | tepid | departname |
+----+-------+--------+-----------+-------+------------+
|  2 | Tom   |  80000 |     80000 |     1 | it         |
|  3 | Mary  |  50000 |     50000 |     2 | kuaiji     |
|  6 | Janet | 780000 |    780000 |     3 | yunwei     |
+----+-------+--------+-----------+-------+------------+

B:
查询那些比本部门平均工资高的员工的信息

mysql> select ss.*, d.dname from (select s.stname, s.stid, s.stsal, s.stdepno, t.avgsal from (select stdepno, avg(stsal) as avgsal from staff group by stdepno) as t inner join staff as s on s.stdepno=t.stdepno where s.stsal > t.avgsal ) as ss inner join department as d on ss.stdepno = d.deptno;

另外一种写法：

mysql> select tt.*, d.dname from (select s.stid, s.stname,s.stsal,temp.avgsal, s.stdepno  from (select stdepno, avg(stsal) as avgsal from staff group by stdepno) as temp , staff as s where temp.stdepno=s.stdepno and s.stsal > temp.avgsal) as tt , department as d where tt.stdepno = d.deptno;

还有另一种简洁的写法：

select s1.* from staff as s1 where s1.stsal > (select avg(stsal) from staff as s2 where s2.stdepno = s1.stdepno );

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