mysql - 有条件地对不同的列进行 GROUP BY

Question

我想显示聊天消息列表。

为此，我有一个 SELECT 语句:

SELECT id, `from`, sent, message, recd FROM chat 
WHERE id IN(
  SELECT MAX(id) FROM chat
  WHERE `to` = ? GROUP BY `from`
)
ORDER BY id DESC

问题是，我想有条件地选择 to = maria 和 from = maria 的消息。

也就是说，如果 to = maria 我想按 from 分组，如果 from = maria，我想按 到。

如何动态更改此GROUP BY？

Answer 1

我会得到一个 UNION。

(SELECT id, 'from' as direction, m_from AS fromto, sent, message, recd
FROM chat WHERE id IN (SELECT MAX(id) FROM chat WHERE to = 'Maria' GROUP BY m_from))
UNION
(SELECT id, 'to' as direction, m_to AS fromto, sent, message, recd FROM chat
WHERE id IN (SELECT MAX(id) FROM chat WHERE m_from = 'Maria' GROUP BY to))

如果您选择 MAX(id)，则不需要 ORDER BY id 语句。

您可以像我对“direction”所做的那样在 SELECT 子句中添加额外的字段。