MYSQL：订单主题按最新评论（评论有主题）

Question

我有一个主题表，我有一个评论表。

主题

ID  TOPIC 
1   Guns
2   School
3   Politics
4   War

评论

DATE       TOPIC     TEXT
5/4/2013   School    'I love school.'
5/5/2013   War       'War sucks.'
5/6/2013   Politics  'I like politics... sometimes.'
5/7/2013   Guns      'I have a few guns. Do you?'
5/8/2013   War       'Stop fighting you guys.'

我需要的是一个MYSQL语句来获得这个结果（按照与该主题相关的最新评论的顺序）：

TOPIC
War
Guns
Politics
School

这是按最新评论排序的主题列表。

Answer 1

一些建议 -

1. 将您的DATE列格式更改为MySQL日期格式YYYY-mm-dd
2. 使您的TOPIC列成为对该主题的主题表ID的引用

鉴于这些更改，您可以使用此查询执行此操作 -

SELECT t.`TOPIC` FROM Comments c 
JOIN Topics t ON t.ID = c.`TOPIC`
GROUP BY c.`TOPIC`
ORDER BY MAX(`DATE`) DESC

你可以看到这个SQLFiddle示例 - http://sqlfiddle.com/#!2/02560/1

Answer 2

SELECT * FROM `table` WHERE `topic` = 'War' ORDER BY `date`
UNION
SELECT * FROM `table` WHERE `topic` = 'Guns' ORDER BY `date`
UNION
SELECT * FROM `table` WHERE `topic` = 'Politics' ORDER BY `date`
UNION
SELECT * FROM `table` WHERE `topic` = 'School' ORDER BY `date`

不是真正的效率之母，但会起作用..

Answer 3

您正在寻找此查询：

SELECT t.topic FROM Comments c 
INNER JOIN Topics t ON t.topic = c.topic
GROUP BY c.topic
ORDER BY c.date DESC

我建议添加以下内容：

• 根据MYSQL更改日期格式。
• 使用主题ID作为参考而不是主题。