如何对来自不同表的汇总列进行分组？

Question

我有一个用户表 (User) 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)。

我想选择教程最多的10个用户和总和 他们的教程的正面/负面评价。

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。我怎样才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的 UNION ALL 进行子查询，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，并且比在执行聚合之前联合所有三个表的性能更好。

问题是 LEFT JOIN 上的 SUMmation 意味着结果可能为 NULL，您不能将其与其他列的总和一起添加。即：

... 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。

您需要使用 COALESCE 将这些值转换为零才能进行数学运算 - IFNULL 是一种可接受的替代方案，但它是 MySQL 特定的，因此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