SQL - JOIN复杂查询

Question

在教程中，我看到这个SQL查询：

SELECT a.id as a_id, a.info as a_info, 
     b.id as b_id, b.info as b_info 
FROM stats AS a 
JOIN stats as b 
ON a.id != b.id 
LIMIT 25 

查询运作良好，但现在我需要添加WHERE条件。我有表users。表stats包含列user_id。 我尝试从表stats如果users.city=1（例如）获得所有用户，但我仍然无法找到一种方法，如何实现这一目标？

我的问题是，我目前还没有线索，如何在查询中放入另一个JOIN（表users）。

我会为每一个建议表示感谢，谢谢

Answer 1

那么，你使用表stats两次在你的查询中，你实际上也需要两个连接users表。 （对于统计A和统计B）

SELECT a.id as a_id, a.info as a_info, 
     b.id as b_id, b.info as b_info 
FROM stats AS a 
JOIN stats AS b ON a.id != b.id 
JOIN users AS a_users ON a.user_id = a_users.id 
JOIN users AS b_users ON b.user_id = b_users.id 
WHERE a_users.city = 1 
    OR b_users.city = 1 
LIMIT 25 

Answer 2

因此，这是你有什么...

这是你回来什么

SELECT a.id as a_id, a.info as a_info, b.id as b_id, b.info as b_info 

这是从何而来从

FROM stats AS a 
JOIN stats as b on a.id != b.id 

这是多少你想

LIMIT 25 

所以，如果你想添加的用户表，其他的以后添加此加入

JOIN users ON a.user_id = users.id 

，并对其进行过滤，加入这个连接

WHERE users.city=1 

发出后

SELECT a.id as a_id, a.info as a_info, b.id as b_id, b.info as b_info 
FROM stats AS a 
JOIN stats as b on a.id != b.id 
JOIN users ON a.user_id = users.id 
WHERE users.city=1 
LIMIT 25 

Answer 3

为了方便起见，我已经重新格式化了原始SQL语句以使其人类可读。

SELECT a.id AS a_id 
    , a.info AS a_info 
    , b.id AS b_id 
    , b.info AS b_info 
    FROM stats a 
    JOIN stats b 
    ON a.id != b.id 
LIMIT 25 

您当前的查询几乎返回笛卡尔积。来自统计数据的每一行都与统计数据中的每一行进行匹配，除了匹配自身。 （假设stats.id是主键或唯一键。）

要添加一个连接到用户表，限制行从a返回，例如：

SELECT a.id AS a_id 
    , a.info AS a_info 
    , b.id AS b_id 
    , b.info AS b_info 
    FROM stats a 
    JOIN users au ON au.id = a.user_id AND au.city=1 
    JOIN stats b ON a.id != b.id 
LIMIT 25 

如果要限制返回的行两个a和b，再添加入到users表：

SELECT a.id AS a_id 
    , a.info AS a_info 
    , b.id AS b_id 
    , b.info AS b_info 
    FROM stats a 
    JOIN users au ON au.id = a.user_id AND au.city=1 
    JOIN stats b ON a.id != b.id 
    JOIN users bu ON bu.id = b.user_id AND bu.city=1 
LIMIT 25 

这是不是为了实现这一目标的唯一途径。例如，您可以使用a.user_id IN (subquery)或​​谓词。

（如果您使用可读的格式进行格式化，SQL更容易使用。）

Answer 4

下面是做到这一点的一种方法：

SELECT a.id as a_id, a.info as a_info, 
     b.id as b_id, b.info as b_info 
FROM stats AS a 
JOIN stats as b 
    ON a.id != b.id 
where a.user_id in (select user_id from users where users.city = 1) 
LIMIT 25 

你也可以使用这个作为连接（这在MySQL是更有效）：

SELECT a.id as a_id, a.info as a_info, 
     b.id as b_id, b.info as b_info 
FROM stats AS a 
JOIN stats AS b 
    ON a.id != b.id 
JOIN users AS u 
    ON a.user_id = u.user_id 
    AND u.city_id = 1 
LIMIT 25