通过 id mysql 查找具有最新日期的行

Question

我有这 2 个表，我正在尝试编写一个查询来帮助我选择所有给出此结果的行

用户

id	name
1	test
2	test2

日志

id	userId	message	date
1	1	this is a test	2020-10-07 12:57:14
2	1	another reason	2020-10-07 13:57:14
3	1	another reason 2	2020-10-07 14:57:14
4	2	another reason 3	2020-10-04 12:57:14
5	2	another reason 4	2020-10-05 12:57:14
6	2	another reason 4	2020-10-06 12:57:14

输出表 我需要像在这种情况下（1,2）一样传递许多用户ID，并在表格下方仅返回每个用户ID每行的MAX（日期）

id	userId	message	date
3	1	another reason 2	2020-10-07 14:57:14
6	2	another reason 4	2020-10-06 12:57:14

这可以通过一个查询实现吗？这是我有但不工作的东西

SELECT 
id ,
userId ,
message,
date
FROM logs
WHERE userId IN (1,2)
ORDER BY date DESC;

Answer 1

您可以使用窗口函数 ROW_NUMBER 的结果来检索这些结果。

SELECT
    id,userId,message,date
FROM (
    SELECT 
        *,
        ROW_NUMBER() OVER (
            PARTITION BY userId
            ORDER BY date DESC
        ) as rn
    FROM 
        logs
) t
WHERE rn=1 AND
      userId IN (1,2)
ORDER BY date DESC

对于旧的 mysql 版本

SELECT
    id,userId,message,date
FROM (
    SELECT 
        l.*,
        @row_num:=IF(userId=@prev_user_id,@row_num+1,1) as rn,
        @prev_user_id:=userId
    FROM 
        logs l
    CROSS JOIN (
        SELECT @row_num:=0, @prev_user_id:=NULL
    ) as vars
    ORDER BY userId, date DESC
    
) t
WHERE rn=1 AND
      userId IN (1,2)
ORDER BY date DESC