我有一张叫做employeesee的桌子（三人小组）：
+------------+------------+---------------+--------------+
| 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 |
+------------+------------+---------------+--------------+
我的意图是在同一个国家内配对（自我加入）人并将它们并排放置。
所以我在同事的帮助下提出了以下问题：
SELECT f.employeeid, f.lastname, s.employeeid, s.lastname, f.country
FROM employeee f INNER JOIN employeee s ON f.country = s.country
结果不是我想要的100％。交叉连接似乎在结果中：
+------------+------------+------------+------------+---------------+
| 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 |
+------------+------------+------------+------------+---------------+
另外，为什么在AS时没有使用f.employeeid, s.employeeid alias关键字？我认为这些是必需的。
答案 0 :(得分：2)
如果您只想在一个国家/地区内拥有唯一的一对人，并且您希望避免将他们与自己配对，请在查询中添加以下条件
WHERE f.employeeid < s.employeeid