MySql的自联接子内连接返回交叉连接

Question

我有一个表称为employeee（三联E公司）：

+------------+------------+---------------+--------------+ 
| employeeid | lastname | country  | departmentid | 
+------------+------------+---------------+--------------+ 
|  123 | Rafferty | Australia  |   31 | 
|  124 | Jones  | Australia  |   33 | 
|  145 | Heisenberg | Australia  |   33 | 
|  201 | Robinson | United States |   34 | 
|  305 | Smith  | Germany  |   34 | 
|  306 | Williams | Germany  |   NULL | 
+------------+------------+---------------+--------------+ 

我的目的是配对（自联接）同一国家内的人并把他们一边侧。

于是我想出了与同事一些帮助下面的查询：

SELECT f.employeeid, f.lastname, s.employeeid, s.lastname, f.country 
FROM employeee f INNER JOIN employeee s ON f.country = s.country 

结果是不是100％，我打算。交叉连接似乎是在结果：

+------------+------------+------------+------------+---------------+ 
| employeeid | lastname | employeeid | lastname | country  | 
+------------+------------+------------+------------+---------------+ 
|  123 | Rafferty |  123 | Rafferty | Australia  | 
|  124 | Jones  |  123 | Rafferty | Australia  | 
|  145 | Heisenberg |  123 | Rafferty | Australia  | 
|  123 | Rafferty |  124 | Jones  | Australia  | 
|  124 | Jones  |  124 | Jones  | Australia  | 
|  145 | Heisenberg |  124 | Jones  | Australia  | 
|  123 | Rafferty |  145 | Heisenberg | Australia  | 
|  124 | Jones  |  145 | Heisenberg | Australia  | 
|  145 | Heisenberg |  145 | Heisenberg | Australia  | 
|  201 | Robinson |  201 | Robinson | United States | 
|  305 | Smith  |  305 | Smith  | Germany  | 
|  306 | Williams |  305 | Smith  | Germany  | 
|  305 | Smith  |  306 | Williams | Germany  | 
|  306 | Williams |  306 | Williams | Germany  | 
+------------+------------+------------+------------+---------------+ 

而且为什么没有做f.employeeid, s.employeeid当使用AS别名关键字？我认为这些都是必需的。

Answer 1

如果你只想要一国之内的人独特的对，你想避免配对的人了自己，然后添加以下条件查询

WHERE f.employeeid < s.employeeid