SQL - JOIN複雜查詢

Question

在教程中，我看到這個SQL查詢：

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 

查詢運作良好，但現在我需要添加WHERE條件。我有表users。表stats包含列user_id。 我嘗試從表stats如果users.city=1（例如）獲得所有用戶，但我仍然無法找到一種方法，如何實現這一目標？

我的問題是，我目前還沒有線索，如何在查詢中放入另一個JOIN（表users）。

我會爲每一個建議表示感謝，謝謝

Answer 1

那麼，你使用表stats兩次在你的查詢中，你實際上也需要兩個連接users表。 （對於統計A和統計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，這是你有什麼...

這是你回來什麼

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

這是從何而來從

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

這是多少你想

LIMIT 25 

所以，如果你想添加的用戶表，其他的以後添加此加入

JOIN users ON a.user_id = users.id 

，並對其進行過濾，加入這個連接

WHERE users.city=1 

發出後

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

爲了方便起見，我已經重新格式化了原始SQL語句以使其人類可讀。

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 

您當前的查詢幾乎返回笛卡爾積。來自統計數據的每一行都與統計數據中的每一行進行匹配，除了匹配自身。 （假設stats.id是主鍵或唯一鍵。）

要添加一個連接到用戶表，限制行從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兩個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 

這是不是爲了實現這一目標的唯一途徑。例如，您可以使用a.user_id IN (subquery)或​​謂詞。

（如果您使用可讀的格式進行格式化，SQL更容易使用。）

Answer 4

下面是做到這一點的一種方法：

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 

你也可以使用這個作爲連接（這在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