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 );
