如何从sqlitedatabase中的表从每个组中获取第一行？

Question

I have a table messages in my sqlite database.This table has three columns msg_id,sender_id,msg_body. I need to get one latest message with each user_id and these messages should be ordered in desc order. for eg. my messages table is

 msg_id | sender_id | msg_body
--------+-----------+----------
   1    |    18     |"something"
   2    |    18     |"something"
   3    |    19     |"something"
   4    |    19     |"something"

What I want is table:-

msg_id | sender_id | msg_body
-------+-----------+----------
  4    |    19     |"something"
  2    |    18     |"something"

Answer 1

One option would be to do an INNER JOIN of your original table with a subquery containing the most recent record for each sender_id group:

SELECT t1.msg_id, t1.sender_id, t1.msg_body
FROM yourTable t1
INNER JOIN
(
    SELECT sender_id, MAX(msg_id) AS max_msg_id
    FROM yourTable
    GROUP BY sender_id
) t2
    ON t1.msg_id = t2.max_msg_id AND t1.sender_id = t2.sender_id

Follow the link below for a running demo:

SQLFiddle

Answer 2

Try this:

SELECT t1.*
FROM mytable AS t1
JOIN (
   SELECT sender_id, MAX(msg_id) AS max_msg_id
   FROM mytable
   GROUP BY sender_id
) AS t2 ON t1.sender_id = t2.sender_id AND t1.msg_id = t2.max_msg_id

Answer 3

最后显示给发件人的消息。

SELECT * FROM messagees group by sender_id order by id msg_id