如何从表格中检索不同的行

Question

我的数据库中有一个名为message的表，用户可以在其中发送和检索消息。

这是布局：

msg_id sender_id receiver_id 日期 txt

我希望能够检索发送给特定用户的消息，每个用户只有最后一条消息，例如，用户ID 1可能有多个消息，但是我只想显示最后一条消息。

Answer 1

http://sqlfiddle.com/#!9/81b57/3

SELECT m.*
FROM messages m
LEFT JOIN messages m1
ON m.sender_id = m1.sender_id
  AND m.date<m1.date
WHERE m1.msg_id IS NULL
ORDER BY sender_id

要么

SELECT m.*
FROM messages m
LEFT JOIN messages m1
ON m.receiver_id = m1.receiver_id
  AND m.date<m1.date
WHERE m1.msg_id IS NULL
ORDER BY receiver_id;

更新这是每个用户记录的灵活限制（2-3-4 ... any）的变体。 但是请记住，如果您有数百万条记录，它可能会变得非常缓慢。

http://sqlfiddle.com/#!9/81b57/13

SELECT m.*
FROM
(SELECT *, 
   IF(@user=receiver_id,@user_index:=@user_index+1,@user_index:=1) idx,
   @user:=receiver_id
FROM messages 
ORDER BY receiver_id, `date` DESC
) m
WHERE m.idx<=2

**更新2 *另一种方法。 我想可能会更快：

http://sqlfiddle.com/#!9/6bfbd/1

SELECT m.*
FROM messages m
INNER JOIN (
  SELECT receiver_id, 
  GROUP_CONCAT(msg_id ORDER BY `date` DESC) ord
  FROM messages 
  GROUP BY receiver_id

) o
ON o.receiver_id = m.receiver_id
  and FIND_IN_SET(m.msg_id, o.ord) BETWEEN 1 AND 2
ORDER BY m.receiver_id, m.date DESC
  ;

Answer 2

您需要对查询使用ORDER BY和LIMIT

例：

SELECT * FROM message WHERE receiver_id=1 ORDER BY date DESC LIMIT 1