MySQL选择最频繁的组

Question

如何获取MySQL中每个标记最常出现的类别？理想情况下,我想模拟一个计算列模式的聚合函数.

SELECT 
  t.tag 
  , s.category 
FROM tags t 
LEFT JOIN stuff s 
USING (id) 
ORDER BY tag;

+------------------+----------+
| tag              | category |
+------------------+----------+
| automotive       |        8 |
| ba               |        8 |
| bamboo           |        8 |
| bamboo           |        8 |
| bamboo           |        8 |
| bamboo           |        8 |
| bamboo           |        8 |
| bamboo           |       10 |
| bamboo           |        8 |
| bamboo           |        9 |
| bamboo           |        8 |
| bamboo           |       10 |
| bamboo           |        8 |
| bamboo           |        9 |
| bamboo           |        8 |
| banana tree      |        8 |
| banana tree      |        8 |
| banana tree      |        8 |
| banana tree      |        8 |
| bath             |        9 |
+-----------------------------+

Answer 1

SELECT t1.*
FROM (SELECT tag, category, COUNT(*) AS count
      FROM tags INNER JOIN stuff USING (id)
      GROUP BY tag, category) t1
LEFT OUTER JOIN 
     (SELECT tag, category, COUNT(*) AS count
      FROM tags INNER JOIN stuff USING (id)
      GROUP BY tag, category) t2
  ON (t1.tag = t2.tag AND (t1.count < t2.count 
      OR t1.count = t2.count AND t1.category < t2.category))
WHERE t2.tag IS NULL
ORDER BY t1.count DESC;

我同意这对于单个 SQL 查询来说有点太多了。GROUP BY任何在子查询内部的使用都会让我畏缩。您可以使用视图使其看起来更简单：

CREATE VIEW count_per_category AS
    SELECT tag, category, COUNT(*) AS count
    FROM tags INNER JOIN stuff USING (id)
    GROUP BY tag, category;

SELECT t1.*
FROM count_per_category t1
LEFT OUTER JOIN count_per_category t2
  ON (t1.tag = t2.tag AND (t1.count < t2.count 
      OR t1.count = t2.count AND t1.category < t2.category))
WHERE t2.tag IS NULL
ORDER BY t1.count DESC;

但它基本上在幕后做同样的工作。

您评论说您可以在应用程序代码中轻松执行类似的操作。那你为什么不这样做呢？执行更简单的查询来获取每个类别的计数：

SELECT tag, category, COUNT(*) AS count
FROM tags INNER JOIN stuff USING (id)
GROUP BY tag, category;

并对应用程序代码中的结果进行排序。