把一張表 想象成兩張表，進行多表查詢
舉例：
- 查詢 所有員工的 姓名 以及 其 上級姓名
select s1.stname, s2.stname from staff as s1, staff as s2 where s1.stmgr = s2.stid;
- 查詢 員工李巖的 上級姓名
select s1.stname, s2.stname from staff as s1, staff as s2 where s1.stmgr = s2.stid and s1.stname='李巖';
以上是 利用表的多表查詢
還可以使用子查詢 來實現
比如：
select staff.stname from staff where stid = (select stmgr from staff where stname='李巖');
復雜一些的表查詢舉例：
A：
有兩張表，分別是員工信息表，與部門信息表，要求查詢，各個部門工資最高的員工的信息
員工表：
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 |
+----+-------+--------+-------+部門表：
mysql> select * from department;
+----+--------+
| id | name |
+----+--------+
| 1 | it |
| 2 | kuaiji |
| 3 | yunwei |
+----+--------+
思考步驟：
1、從employee表里查詢出每個部門的最高薪資，作為一張臨時表 t
mysql> select depid , max(salary) as maxsalary from employee group by depid;
+-------+-----------+
| depid | maxsalary |
+-------+-----------+
| 1 | 80000 |
| 2 | 50000 |
| 3 | 780000 |
+-------+-----------+2、將臨時表 t 和 employee表進行內連接，并新增一列，顯示employee表里每個員工所在部門對應的最高薪資
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表里每個員工的薪資字段salary 和 部門最高薪資字段列maxsalary進行判斷，查詢出相等數據，此處則查詢出了每個部門最高薪資的員工有哪些，作為表 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、因為表 tt 沒有部門名稱，所以我們再將表 tt 和department 表進行內鏈接，查詢部門id相等的數據，從而查詢出每個員工所在的部門名稱
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:
查詢 那些 比本部門 平均工資 高的員工的信息
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;
另外一種寫法：
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;
還有另一種簡潔的寫法：
select s1.* from staff as s1 where s1.stsal > (select avg(stsal) from staff as s2 where s2.stdepno = s1.stdepno );
