Question

我需要在这张桌子上使用自我加入。

+------------+------+--------+
| Country    | Rank |  Year  |
+------------+------+--------+
|France      |  55  |  2000  |
+------------+------+--------+
|Canada      |  30  |  2000  |
+------------+------+--------+ 
|Liberia     |  59  |  2001  |
+------------+------+--------+ 
|Turkey      |  78  |  2000  |
+------------+------+--------+ 
|Japan       |  65  |  2003  |
+------------+------+--------+
|Romania     |  107 |  2001  |
+------------+------+--------+

我需要使用自我加入来获得与土耳其同年的国家。仅显示国家/地区和年份。

这就是我想要做的。

SELECT DISTINCT a.Country, a.Year 
FROM table1 AS a, table1 AS b 
WHERE a.Year=b.Year and a.Country='Turkey';

^ googled self join，并成功。

我只得到土耳其。我做错了什么？

Answer 1

你真是太近了！

既然你说你正在显示来自A的国家和年份并且被土耳其A. Country限制，那么土耳其就是你要看到的。您需要将选择更改为B.country和B.year，或将where子句更改为B.country。

这是使用交叉连接，这会使表中的记录越多越慢。

SELECT DISTINCT b.Country, b.Year 
FROM table1 AS a, 
     table1 AS b 
WHERE a.Year=b.Year 
  and a.Country='Turkey';

可以写成......并且可能会有相同的执行计划。

SELECT DISTINCT b.Country, b.Year 
FROM table1 AS a 
CROSS JOIN table1 AS b 
WHERE a.Year=b.Year 
  and a.Country='Turkey';

OR 这使用INNER JOIN来限制引擎必须执行的工作，并且不会受到交叉连接的性能下降的影响。

SELECT DISTINCT a.Country, a.Year 
FROM table1 AS a 
INNER JOIN table1 AS b 
   on a.Year=b.Year 
  and b.Country='Turkey';

为什么：

考虑连接发生时SQL引擎将执行的操作 A B

+------------+------+--------+------------+------+--------+
| A.Country  | Rank |  Year  | B.Country  | Rank |  Year  |
+------------+------+--------+------------+------+--------+
|France      |  55  |  2000  |France      |  55  |  2000  |
+------------+------+--------+------------+------+--------+
|Canada      |  30  |  2000  |France      |  55  |  2000  |
+------------+------+--------+------------+------+--------+ 
|Turkey      |  78  |  2000  |France      |  55  |  2000  |
+------------+------+--------+------------+------+--------+ 
|France      |  55  |  2000  |Canada      |  30  |  2000  |
+------------+------+--------+------------+------+--------+
|Canada      |  30  |  2000  |Canada      |  30  |  2000  |
+------------+------+--------+------------+------+--------+ 
|Turkey      |  78  |  2000  |Canada      |  30  |  2000  |
+------------+------+--------+------------+------+--------+ 
|France      |  55  |  2000  |Turkey      |  78  |  2000  |
+------------+------+--------+------------+------+--------+
|Canada      |  30  |  2000  |Turkey      |  78  |  2000  |
+------------+------+--------+------------+------+--------+ 
|Turkey      |  78  |  2000  |Turkey      |  78  |  2000  |
+------------+------+--------+------------+------+--------+

所以当你说显示A.Country和A.Year A.Country是土耳其时，你可以看到它可以返回的是土耳其（由于仅有1条记录）

但是如果你B.Country是土耳其并展示A.Country，那么你将获得法国，加拿大和土耳其！

Answer 2

将a.Country = 'Turkey'更改为b.Country = 'Turkey'

您有SELECT DISTINCT a.Country，但您的条件是a.Country = 'Turkey'。即使您确实获得了多行，也会按DISTINCT

进行过滤

Answer 3

select distinct country,year from table1 where year=(select year from table  
where country='turkey') and country !=turkey;

MySQL的。如何使用自我加入

3 个答案: