Question

我有一个用户表（用户）和3个教程表（文本，视频和其他）。

每个教程都有rating_positive和rating_negative列并链接到用户（id）。

我想选择教程和总和最多的10个用户他们的教程的正/负评级。

我尝试了以下查询，但它不起作用。它为tutorials_count / pos / neg返回太多结果。我怎么能正确地做到这一点？

SELECT  
    u.id AS user_id,  
    (COUNT(t.id) + COUNT(v.id) + COUNT(o.id)) AS tutorials_count,
    (SUM(t.rating_positive) + SUM(v.rating_positive) + SUM(o.rating_positive)) AS pos,
    (SUM(t.rating_negative) + SUM(v.rating_negative) + SUM(o.rating_negative)) AS neg
FROM
    user u LEFT JOIN trick t ON u.id = t.submitter_id
    LEFT JOIN video v ON u.id = v.submitter_id
    LEFT JOIN other o ON u.id = o.submitter_id
GROUP BY u.id
ORDER BY tutorials_count DESC
LIMIT 10

Answer 1

尝试使用您感兴趣的三个表中的UNION ALL创建子查询，然后加入：

SELECT  
    u.id AS user_id,  
    COUNT(submitter_id) AS tutorials_count,
    IFNULL(SUM(rating_positive), 0) AS pos,
    IFNULL(SUM(rating_negative), 0) AS neg
FROM user u
LEFT JOIN (
    SELECT submitter_id, rating_positive, rating_negative FROM trick
    UNION ALL
    SELECT submitter_id, rating_positive, rating_negative FROM video
    UNION ALL
    SELECT submitter_id, rating_positive, rating_negative FROM other
) T1
ON u.id = T1.submitter_id
GROUP BY u.id
ORDER BY tutorials_count DESC
LIMIT 10

Answer 2

LEFT JOIN很好，并且在执行聚合之前执行比联合所有三个表更好。

问题是LEFT JOIN上的SUMmation意味着结果可能为NULL，您无法将其与其他列的总和一起添加。 IE：

... SUM(t.rating_positive) + 1

如果SUM(t.rating_positive)没有支持记录，

...将返回NULL，因为NULL + 1等于NULL。

你需要使用COALESCE将这些转换为零以使数学工作 - IFNULL是一个可接受的选择，但它是MySQL特定的，所以不太可能是可移植的：

   SELECT u.id AS user_id,  
          COALESCE(COUNT(t.id), 0) + COALESCE(COUNT(v.id), 0) + COALESCE(COUNT(o.id), 0) AS tutorials_count,
          COALESCE(SUM(t.rating_positive), 0) + COALESCE(SUM(v.rating_positive), 0) + COALESCE(SUM(o.rating_positive), 0) AS pos,
          COALESCE(SUM(t.rating_negative), 0) + COALESCE(SUM(v.rating_negative), 0) + COALESCE(SUM(o.rating_negative), 0) AS neg
     FROM USER u 
LEFT JOIN trick t ON u.id = t.submitter_id
LEFT JOIN video v ON u.id = v.submitter_id
LEFT JOIN other o ON u.id = o.submitter_id
 GROUP BY u.id
 ORDER BY tutorials_count DESC
    LIMIT 10

如何对来自不同表的总结列进行分组？

2 个答案: