mysql 获取分组中最新记录SQL写法示例select group by order by

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xxx.infrastructure.core.version.dao.VersionDAO">
    <select id="batchQueryTopVersionList"
            parameterType="com.xxx.infrastructure.api.version.param.VersionQueryParam"
            resultMap="com.xxx.infrastructure.core.version.dao.base.BaseVersionDAO.ResultMapWithBLOBs">

        select a.* from
        (select * from version WHERE is_deleted = 'n'
        and biz_type = #{versionQueryParam.bizType}
        and domain = #{versionQueryParam.domain}
        <if test="versionQueryParam.bizIdList != null and versionQueryParam.bizIdList.size > 0">
            and biz_id in
            <foreach collection="versionQueryParam.bizIdList" separator="," index="index" open="(" close=")"
                     item="item">
                #{item}
            </foreach>
        </if>
        ) a
        join
        (SELECT
        biz_id,
        history_id,
        max(gmt_create) as gmt_create
        FROM version
        WHERE is_deleted = 'n'
        and biz_type = #{versionQueryParam.bizType}
        and domain = #{versionQueryParam.domain}
        <if test="versionQueryParam.bizIdList != null and versionQueryParam.bizIdList.size > 0">
            and biz_id in
            <foreach collection="versionQueryParam.bizIdList" separator="," index="index" open="(" close=")"
                     item="item">
                #{item}
            </foreach>
        </if>
        group by biz_id) b
        on concat(a.biz_id,a.gmt_create) = concat(b.biz_id,b.gmt_create)
    </select>
</mapper>

I have a mysql statement

SELECT * 
FROM tbl_messages 
WHERE to_user_id = '$user_id' OR from_user_id = '$user_id' 
GROUP BY from_user_id 
ORDER BY date_sent DESC

and it is producing the correct results however they are not in the correct order.

The grouping works well but it record displayed in the group is the first recorded entered into the DB but I would like the latest record to be displayed in each group.

Is there a way to have the latest record displayed for each group

2011-12-19 12:16:25 This is the first message
2011-12-19 12:18:20 This is the second message
2011-12-19 12:43:04 This is the third message

The group shows 'This is the first message' where I would like 'This is the third message' as that is the most recent record/message.

Cheers.

This may work (but not guaranteed):

SELECT * 
FROM
  ( SELECT *
    FROM tbl_messages 
    WHERE to_user_id = '$user_id' OR from_user_id = '$user_id' 
    ORDER BY date_sent DESC
  ) tmp
GROUP BY from_user_id 
ORDER BY date_sent DESC

This should work:

SELECT t.* 
FROM 
    tbl_messages AS t
  JOIN
    ( SELECT from_user_id 
           , MAX(date_sent) AS max_date_sent
      FROM tbl_messages 
      WHERE to_user_id = '$user_id' OR from_user_id = '$user_id' 
      GROUP BY from_user_id 
    ) AS tg
    ON  (tg.from_user_id, tg.max_date_sent) = (t.from_user_id, t.date_sent)
ORDER BY t.date_sent DESC

参考资料

https://stackoverflow.com/questions/8556283/mysql-select-group-by-order

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