我是自我加入的新手，并编写了以下示例表：
+-----------+-------------+
| name | location |
+-----------+-------------+
| Robert | Guadalajara |
| Manuel | Guadalajara |
| Dalia | Guadalajara |
| Alejandra | Guadalajara |
| Luis | Guadalajara |
| Monica | Guadalajara |
| Claudia | Guadalajara |
| Scartlet | Guadalajara |
| Sergio | Guadalajara |
| Rick | Mexico City |
| Rene | Mexico City |
| Ramon | Culiacan |
| Junior | Culiacan |
| Kasandra | Culiacan |
| Emma | Culiacan |
| Johnatha | Dunedin |
| Miriam | Largo |
| Julie | Largo |
+-----------+-------------+
18行
我想运行一个简单的查询，使用以下查询匹配具有相同位置的人：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1, users users2
WHERE users1.location = users2.location;
我的期望：
+-----------+-------------++-----------+-------------+
| name | location || name | location |
+-----------+-------------++-----------+-------------+
| Robert | Guadalajara || Robert | Guadalajara |
| Manuel | Guadalajara || Manuel | Guadalajara |
| Dalia | Guadalajara || Dalia | Guadalajara |
| Alejandra | Guadalajara || Alejandra | Guadalajara |
| Luis | Guadalajara || Luis | Guadalajara |
| Monica | Guadalajara || Monica | Guadalajara |
| Claudia | Guadalajara || Claudia | Guadalajara |
| Scartlet | Guadalajara || Scartlet | Guadalajara |
| Sergio | Guadalajara || Sergio | Guadalajara |
| Rick | Mexico City || Rick | Mexico City |
| Rene | Mexico City || Rene | Mexico City |
| Ramon | Culiacan || Ramon | Culiacan |
| Junior | Culiacan || Junior | Culiacan |
| Kasandra | Culiacan || Kasandra | Culiacan |
| Emma | Culiacan || Emma | Culiacan |
| Johnatha | Dunedin || Johnatha | Dunedin |
| Miriam | Largo || Miriam | Largo |
| Julie | Largo || Julie | Largo |
+-----------+-------------++-----------+-------------+
我得到了什么：
+-----------+-------------+-----------+-------------+
| name | location | name | location |
+-----------+-------------+-----------+-------------+
| Robert | Guadalajara | Robert | Guadalajara |
| Manuel | Guadalajara | Robert | Guadalajara |
| Dalia | Guadalajara | Robert | Guadalajara |
| Alejandra | Guadalajara | Robert | Guadalajara |
| Luis | Guadalajara | Robert | Guadalajara |
| Monica | Guadalajara | Robert | Guadalajara |
| Claudia | Guadalajara | Robert | Guadalajara |
| Scartlet | Guadalajara | Robert | Guadalajara |
| Sergio | Guadalajara | Robert | Guadalajara |
| Robert | Guadalajara | Manuel | Guadalajara |
| Manuel | Guadalajara | Manuel | Guadalajara |
| Dalia | Guadalajara | Manuel | Guadalajara |
| Alejandra | Guadalajara | Manuel | Guadalajara |
| Luis | Guadalajara | Manuel | Guadalajara |
| Monica | Guadalajara | Manuel | Guadalajara |
| Claudia | Guadalajara | Manuel | Guadalajara |
| Scartlet | Guadalajara | Manuel | Guadalajara |
| Sergio | Guadalajara | Manuel | Guadalajara |
| Robert | Guadalajara | Dalia | Guadalajara |
| Manuel | Guadalajara | Dalia | Guadalajara |
| Dalia | Guadalajara | Dalia | Guadalajara |
| Alejandra | Guadalajara | Dalia | Guadalajara |
| Luis | Guadalajara | Dalia | Guadalajara |
| Monica | Guadalajara | Dalia | Guadalajara |
| Claudia | Guadalajara | Dalia | Guadalajara |
| Scartlet | Guadalajara | Dalia | Guadalajara |
| Sergio | Guadalajara | Dalia | Guadalajara |
| Robert | Guadalajara | Alejandra | Guadalajara |
| Manuel | Guadalajara | Alejandra | Guadalajara |
| Dalia | Guadalajara | Alejandra | Guadalajara |
| Alejandra | Guadalajara | Alejandra | Guadalajara |
| Luis | Guadalajara | Alejandra | Guadalajara |
| Monica | Guadalajara | Alejandra | Guadalajara |
| Claudia | Guadalajara | Alejandra | Guadalajara |
| Scartlet | Guadalajara | Alejandra | Guadalajara |
| Sergio | Guadalajara | Alejandra | Guadalajara |
| Robert | Guadalajara | Luis | Guadalajara |
| Manuel | Guadalajara | Luis | Guadalajara |
| Dalia | Guadalajara | Luis | Guadalajara |
| Alejandra | Guadalajara | Luis | Guadalajara |
| Luis | Guadalajara | Luis | Guadalajara |
| Monica | Guadalajara | Luis | Guadalajara |
| Claudia | Guadalajara | Luis | Guadalajara |
| Scartlet | Guadalajara | Luis | Guadalajara |
| Sergio | Guadalajara | Luis | Guadalajara |
| Robert | Guadalajara | Monica | Guadalajara |
| Manuel | Guadalajara | Monica | Guadalajara |
| Dalia | Guadalajara | Monica | Guadalajara |
| Alejandra | Guadalajara | Monica | Guadalajara |
| Luis | Guadalajara | Monica | Guadalajara |
| Monica | Guadalajara | Monica | Guadalajara |
| Claudia | Guadalajara | Monica | Guadalajara |
| Scartlet | Guadalajara | Monica | Guadalajara |
| Sergio | Guadalajara | Monica | Guadalajara |
| Robert | Guadalajara | Claudia | Guadalajara |
| Manuel | Guadalajara | Claudia | Guadalajara |
| Dalia | Guadalajara | Claudia | Guadalajara |
| Alejandra | Guadalajara | Claudia | Guadalajara |
| Luis | Guadalajara | Claudia | Guadalajara |
| Monica | Guadalajara | Claudia | Guadalajara |
| Claudia | Guadalajara | Claudia | Guadalajara |
| Scartlet | Guadalajara | Claudia | Guadalajara |
| Sergio | Guadalajara | Claudia | Guadalajara |
| Robert | Guadalajara | Scartlet | Guadalajara |
| Manuel | Guadalajara | Scartlet | Guadalajara |
| Dalia | Guadalajara | Scartlet | Guadalajara |
| Alejandra | Guadalajara | Scartlet | Guadalajara |
| Luis | Guadalajara | Scartlet | Guadalajara |
| Monica | Guadalajara | Scartlet | Guadalajara |
| Claudia | Guadalajara | Scartlet | Guadalajara |
| Scartlet | Guadalajara | Scartlet | Guadalajara |
| Sergio | Guadalajara | Scartlet | Guadalajara |
| Robert | Guadalajara | Sergio | Guadalajara |
| Manuel | Guadalajara | Sergio | Guadalajara |
| Dalia | Guadalajara | Sergio | Guadalajara |
| Alejandra | Guadalajara | Sergio | Guadalajara |
| Luis | Guadalajara | Sergio | Guadalajara |
| Monica | Guadalajara | Sergio | Guadalajara |
| Claudia | Guadalajara | Sergio | Guadalajara |
| Scartlet | Guadalajara | Sergio | Guadalajara |
| Sergio | Guadalajara | Sergio | Guadalajara |
| Rick | Mexico City | Rick | Mexico City |
| Rene | Mexico City | Rick | Mexico City |
| Rick | Mexico City | Rene | Mexico City |
| Rene | Mexico City | Rene | Mexico City |
| Ramon | Culiacan | Ramon | Culiacan |
| Junior | Culiacan | Ramon | Culiacan |
| Kasandra | Culiacan | Ramon | Culiacan |
| Emma | Culiacan | Ramon | Culiacan |
| Ramon | Culiacan | Junior | Culiacan |
| Junior | Culiacan | Junior | Culiacan |
| Kasandra | Culiacan | Junior | Culiacan |
| Emma | Culiacan | Junior | Culiacan |
| Ramon | Culiacan | Kasandra | Culiacan |
| Junior | Culiacan | Kasandra | Culiacan |
| Kasandra | Culiacan | Kasandra | Culiacan |
| Emma | Culiacan | Kasandra | Culiacan |
| Ramon | Culiacan | Emma | Culiacan |
| Junior | Culiacan | Emma | Culiacan |
| Kasandra | Culiacan | Emma | Culiacan |
| Emma | Culiacan | Emma | Culiacan |
| Johnatha | Dunedin | Johnatha | Dunedin |
| Miriam | Largo | Miriam | Largo |
| Julie | Largo | Miriam | Largo |
| Miriam | Largo | Julie | Largo |
| Julie | Largo | Julie | Largo |
+-----------+-------------+-----------+-------------+
有人可以向我解释为什么我从建议的解决方案中得到这个结果？我想知道这是如何以及为什么会产生的。
感谢。
答案 0 :(得分：2)
您在位置匹配，多次出现。这将乘以您当前的结果。
您的预期结果是姓名加入：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1
INNER JOIN users users2 ON users1.name = users2.name;
查询的工作原理：
在数据集中，WHERE中的列是匹配的。这等同于JOIN：
users1.name现在更常用的语法是哪种。两个查询都是相同的，但是通过JOIN，它变得更具可读性和灵活性。
JOIN中的匹配是在users2.name等于INNER users users2 ON users1.location = users2.location;
的情况下进行的。然后显示找到的结果：
|罗伯特|瓜达拉哈拉|
例如。
如果使用location而不是name运行查询：
+-----------+-------------+----+
| name | location | ID |
+-----------+-------------+----+
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 2 |
SQL匹配位置。因此，对于更短的更易读的例子，想象一下这个短表。添加列ID以显示结果的收集方式：
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 1 |
这在位置匹配，发生两次：
Id 1与Id 1和Id 2（使用位置）匹配：导致2个位置记录：
| Robert | Guadalajara | 2 |
| Manuel | Guadalajara | 2 |
Id 2与Id 1和Id 2（使用位置）匹配：导致2个位置记录：
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 1 |
| Robert | Guadalajara | 2 |
| Manuel | Guadalajara | 2 |
那么结果将是：
grep因此你的原始记录会倍增。
答案 1 :(得分：2)
要获得您期望的结果，您需要加入名称和位置：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1
JOIN users users2 ON users1.location = users2.location
AND users1.name = users2.name ;
在问题中你说你希望结果匹配具有相同位置的人，如果是这种情况，那么你想要的是：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1
JOIN users users2 ON users1.location = users2.location
AND users1.name <> users2.name ;
有关结果的示例，请参阅此sample SQL Fiddle
请注意，第二个查询将复制结果，因为每个匹配都会有相应的对称匹配（Robert和Luis匹配，但也有Luis和Robert）。我认为以下查询应该解决这个问题：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1
JOIN users users2 ON users1.location = users2.location
AND CASE WHEN users1.name < users2.name THEN users1.name ELSE users2.name END <> users2.name;
Sample SQL Fiddle用于上次查询。