MySQL的 - 从双重嵌套子查询

查询我真的需要执行是引用外部表值如下：MySQL的 - 从双重嵌套子查询

SELECT  u.points 
     (SELECT COUNT(1) FROM (SELECT 1 FROM checkin c INNER JOIN wineries w ON w.id = c.winery_id WHERE c.user_id = u.id GROUP BY region_id) b) as states_visited 
FROM  users u 
GROUP BY u.id 
ORDER BY points DESC

然而，这将导致以下错误：

未知列在“u.id” “where子句”

我试着与用户定义的变量，没有错误，但它实际上没有引用用户定义的变量值因为某些原因：

SELECT  @uid := u.id, u.points 
     (SELECT COUNT(1) FROM (SELECT 1 FROM checkin c INNER JOIN wineries w ON w.id = c.winery_id WHERE c.user_id = @uid GROUP BY region_id) b) as states_visited 
FROM  users u 
GROUP BY u.id 
ORDER BY points DESC

任何想法如何让这项工作？没有明显的诉诸两个单独的查询？

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2013-11-26 Phil LaNasa

你能否简单解释一下正在使用的表格，它们的模式以及您实际试图在查询结果中获得的内容？ –

你可以在sqlFiddle.com上设置一些测试数据，并解释你期望的结果 –

为什么你需要双重嵌套？而如果id是表user的主键，你不需要GROUP BY之一：

SELECT  u.points, 
     (SELECT COUNT(1) FROM reviews WHERE user_id = u.id) AS review_count 
FROM  users AS u 
-- GROUP BY u.id 
ORDER BY points DESC ;

你也GROUP BY在派生表 - 然后加入：

SELECT  u.points, 
      COALESCE(r.review_count,0) AS review_count 
FROM  users AS u 
    LEFT JOIN 
     (SELECT user_id, COUNT(1) AS review_count 
     FROM reviews 
     GROUP BY user_id 
     ) AS r 
     ON r.user_id = u.id 
ORDER BY points DESC ;

或加入然后GROUP BY：

SELECT  u.points, 
      COUNT(r.user_id) AS review_count 
FROM  users AS u 
    LEFT JOIN 
      reviews AS r 
     ON r.user_id = u.id 
GROUP BY u.id, u.points 
ORDER BY points DESC ;

编辑后的版本是困难，但可以在不双重嵌套来完成，也：

SELECT  u.points, 
     (SELECT COUNT(DISTINCT region_id) 
     FROM checkin c INNER JOIN wineries w ON w.id = c.winery_id 
     WHERE c.user_id = u.id 
     ) AS states_visited 
FROM  users u 
ORDER BY points DESC ;

来源

2013-11-26 19:41:36

道歉，我试图在发布之前简化我的查询，但是我花了很多精力！请参阅修订后的子查询，该子查询本身具有创建多行的连接。 –

好的，确实很难。删除双重嵌套和内部的'GROUP BY'并使用'COUNT（DISTINCT region_id）' –

您先生，是一位天才！我喜欢真正降低查询复杂度的解决方案。再次感谢。 –

好像你可以做你想做什么用simpe表连接

SELECT u.id AS `id`, u.points AS `points`, COUNT(r.review_id) AS `review_count` /* or whatever the id record for reviews table is */ 
FROM users AS u 
INNER JOIN reviews AS r 
    ON u.id = r.user_id 
GROUP BY `id` 
ORDER BY `points` DESC

来源

2013-11-26 19:44:08

感谢您的回复，对不起，我不得不进行编辑，请在ypercube的回复中查看评论。 –

@PhilL我还是不明白你为什么需要一个子查询。如果在你的问题中，你描述你的表格，以及你试图在你的查询中得到什么，这可能会有所帮助。 –

SELECT users.id, users.points, count(*) 
FROM users, reviews 
WHERE users.id = reviews.user_id 
GROUP BY id,points 
ORDER BY points DESC

sqlFiddle example

来源

2013-11-26 19:53:59

MySQL的 - 从双重嵌套子查询

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