如何从表格中检索不同的行

Question

I have a table in my database called message where users can send and retrieve messages.

Here is the layout:

msg_id sender_id receiver_id date txt

I want to be able to retrieve messages sent to a particular user, with only the last message for each user eg There might be more than one Message from user id 1 , but I only want to display the last message.

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

OR

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;

UPDATE Here is variant with flexible limit (2-3-4 ...any) of records per user. But keep in mind it could become very slow if you have millions of records.

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

**UPDATE 2* One another approach. I guess it could be faster:

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

You need use ORDER BY and LIMIT for your query

Example:

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