如何从sqlitedatabase中的表从每个组中获取第一行？

Question

我的sqlite数据库中有一个表消息。该表具有三列msg_id，sender_id，msg_body。 我需要获取每个user_id的最新消息，并且这些消息应按desc顺序排序。 例如 我的消息表是

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

我想要的是桌子：-

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

Answer 1

一种选择是使用包含每个sender_id组的最新记录的子查询对原始表进行INNER JOIN ：

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

请点击以下链接以获取正在运行的演示：

SQLFiddle

Answer 2

尝试这个：

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