MySql的自聯接子內連接返回交叉連接

Question

我有一個表稱爲employeee（三聯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 | 
+------------+------------+---------------+--------------+ 

我的目的是配對（自聯接）同一國家內的人並把他們一邊側。

於是我想出了與同事一些幫助下面的查詢：

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

結果是不是100％，我打算。交叉連接似乎是在結果：

+------------+------------+------------+------------+---------------+ 
| 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  | 
+------------+------------+------------+------------+---------------+ 

而且爲什麼沒有做f.employeeid, s.employeeid當使用AS別名關鍵字？我認爲這些都是必需的。

Answer 1

如果你只想要一國之內的人獨特的對，你想避免配對的人了自己，然後添加以下條件查詢

WHERE f.employeeid < s.employeeid