mysql - 如何编写涉及联接的简单sql语句？

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。
如何编写一个sql语句，返回：每个类别的最新主题加上类别名称？??
到目前为止，我所拥有的是：

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;

它从同一个类别返回多个主题，其中每个类别只需要一个主题。

最佳答案

当您试图获取greatest-n-per-group时，需要连接回原始表（topics）以选择具有最大日期的行。因为在子查询中只提到MAX(topic_date)不会使其他列来自找到最大日期的行。如果你还提到MIN(topic_date)？
这应该是一个解决方案（尽管我没有测试过）：

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