自连接返回比预期更多的行
Self join returning more rows than expected
我不熟悉自连接并编写了以下示例table:
+-----------+-------------+
| 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，将具有相同位置的人与以下查询相匹配：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1, users users2
WHERE users1.location = users2.location;
我的预期：
+-----------+-------------++-----------+-------------+
| 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 |
+-----------+-------------+-----------+-------------+
除了建议的解决方案之外，有人可以向我解释为什么我会得到这个结果吗？我想知道这是怎么发生的，为什么会这样。
谢谢。
您在位置上匹配，出现了多次。这与您当前的结果相乘。
您的预期结果是连接名称：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1, users users2
WHERE users1.name = users2.name;
查询的工作原理：
在数据集中，WHERE 中的列是匹配的。这相当于 JOIN:
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1
INNER JOIN users users2 ON users1.name = users2.name;
这是现在比较常用的语法。这两个查询是相同的，但是使用 JOIN 后它变得更具可读性和灵活性。
JOIN 中的匹配是在 users1.name 等于 users2.name 时进行的。然后显示找到的结果：
| Robert | Guadalajara |
例如
如果查询是 运行，位置而不是名称：
INNER users users2 ON users1.location = users2.location;
SQL 匹配位置。因此，对于一个更短、更易读的例子，想象一下这个简短的 table。添加列 ID 以显示收集结果的方式：
+-----------+-------------+----+
| name | location | ID |
+-----------+-------------+----+
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 2 |
这是在位置上匹配的，出现了两次：
Id 1 is matched with Id 1 and Id 2 (using location): Resulting in 2 records for location:
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 1 |
Id 2 is matched with Id 1 and Id 2 (using location): Resulting in 2 records for location:
| Robert | Guadalajara | 2 |
| Manuel | Guadalajara | 2 |
所以最终结果将是：
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 1 |
| Robert | Guadalajara | 2 |
| Manuel | Guadalajara | 2 |
因此您的原始记录成倍增加。
要获得您期望的结果，您需要在名称和位置上加入：
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 ;
在这个问题中，你说你想要一个结果匹配具有相同位置的人，如果是这样，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 ;
查看此 sample SQL Fiddle 以获得两个结果的示例
请注意，第二个查询将重复结果，因为每个匹配项都会有一个相应的对称匹配项（Robert 和 Luis 匹配项，还有 Luis 和 Robert）。我认为以下查询应该可以解决这个问题：
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 最后一个查询。
我不熟悉自连接并编写了以下示例table:
+-----------+-------------+
| 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，将具有相同位置的人与以下查询相匹配：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1, users users2
WHERE users1.location = users2.location;
我的预期：
+-----------+-------------++-----------+-------------+
| 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 |
+-----------+-------------+-----------+-------------+
除了建议的解决方案之外，有人可以向我解释为什么我会得到这个结果吗？我想知道这是怎么发生的，为什么会这样。
谢谢。
您在位置上匹配，出现了多次。这与您当前的结果相乘。
您的预期结果是连接名称：
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1, users users2
WHERE users1.name = users2.name;
查询的工作原理：
在数据集中，WHERE 中的列是匹配的。这相当于 JOIN:
SELECT users1.name, users1.location, users2.name, users2.location
FROM users users1
INNER JOIN users users2 ON users1.name = users2.name;
这是现在比较常用的语法。这两个查询是相同的，但是使用 JOIN 后它变得更具可读性和灵活性。
JOIN 中的匹配是在 users1.name 等于 users2.name 时进行的。然后显示找到的结果：
| Robert | Guadalajara |
例如
如果查询是 运行，位置而不是名称：
INNER users users2 ON users1.location = users2.location;
SQL 匹配位置。因此，对于一个更短、更易读的例子，想象一下这个简短的 table。添加列 ID 以显示收集结果的方式：
+-----------+-------------+----+
| name | location | ID |
+-----------+-------------+----+
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 2 |
这是在位置上匹配的，出现了两次：
Id 1 is matched with Id 1 and Id 2 (using location): Resulting in 2 records for location:
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 1 |
Id 2 is matched with Id 1 and Id 2 (using location): Resulting in 2 records for location:
| Robert | Guadalajara | 2 |
| Manuel | Guadalajara | 2 |
所以最终结果将是：
| Robert | Guadalajara | 1 |
| Manuel | Guadalajara | 1 |
| Robert | Guadalajara | 2 |
| Manuel | Guadalajara | 2 |
因此您的原始记录成倍增加。
要获得您期望的结果，您需要在名称和位置上加入：
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 ;
在这个问题中，你说你想要一个结果匹配具有相同位置的人，如果是这样，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 ;
查看此 sample SQL Fiddle 以获得两个结果的示例
请注意，第二个查询将重复结果，因为每个匹配项都会有一个相应的对称匹配项（Robert 和 Luis 匹配项，还有 Luis 和 Robert）。我认为以下查询应该可以解决这个问题：
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 最后一个查询。