mysql - 我如何编写这个涉及连接的简单 sql 语句？

Question

类别表:

+-----------------+--------------+------+-----+---------+----------------+
| Field           | Type         | Null | Key | Default | Extra          |
+-----------------+--------------+------+-----+---------+----------------+
| cat_id          | int(8)       | NO   | PRI | NULL    | auto_increment |
| cat_name        | varchar(255) | NO   | UNI | NULL    |                |
| cat_description | varchar(255) | NO   |     | NULL    |                |
| cat_order       | int(8)       | YES  |     | NULL    |                |
+-----------------+--------------+------+-----+---------+----------------+

主题表:

+---------------+--------------+------+-----+---------+----------------+
| Field         | Type         | Null | Key | Default | Extra          |
+---------------+--------------+------+-----+---------+----------------+
| topic_id      | int(8)       | NO   | PRI | NULL    | auto_increment |
| topic_subject | varchar(255) | NO   |     | NULL    |                |
| topic_date    | datetime     | NO   |     | NULL    |                |
| topic_cat     | int(8)       | NO   | MUL | NULL    |                |
| topic_by      | int(8)       | NO   | MUL | NULL    |                |
+---------------+--------------+------+-----+---------+----------------+

topic_cat 对应外键关系中的cat_id。

我如何编写一个返回的 sql 语句:每个类别的最新主题加上类别名称？？？

这是我目前所拥有的:

SELECT * FROM categories 
LEFT JOIN
   (SELECT topic_id, topic_subject, max(topic_date) AS MaxDate, topic_cat, topic_by 
   FROM topics 
   GROUP BY topic_subject) AS mt 
ON categories.cat_id = mt.topic_cat 
ORDER BY cat_order;

它返回同一类别的多个主题，而我只希望每个类别有一个主题。

Answer 1

当您尝试获取 greatest-n-per-group ，您需要连接回原始表 (topics) 以选择具有最大日期的行。因为仅在子查询中提及 MAX(topic_date) 不会使 other 列来自找到最大日期的行。如果您还提到 MIN(topic_date) 怎么办？

这应该是一个解决方案(虽然我还没有测试过):

SELECT *
FROM topics AS t
JOIN (SELECT topic_cat, MAX(topic_date) AS topic_date
      FROM topics
      GROUP BY topic_cat) AS maxt USING (topic_cat, topic_date)
RIGHT JOIN categories AS c ON t.topic_cat = c.cat_id