MySQL 为其中的每个 id 获取最新的 3 行

Question

我有以下查询，据说它会为每个 update_id 获取最新的 3 条评论。它似乎有效，直到我发现一个逻辑问题，无论更新如何，它都会获得最新的 3 条评论；它应该为每个 update_id 获取最新的 3 条评论。我怎样才能做到这一点？显然 limit(3) 是错误的。

SELECT  `comms` . * ,  `usr`.`name` 
FROM (
`comms`
)
JOIN  `users` AS usr ON  `usr`.`id` =  `comms`.`user_id` 
WHERE  `update_id` 
IN (
'1451',  '1416',  '1186',  '1157',  '1150',  '1122',  '1057',  '914',  '850',  '816',  '794',  '790',  '749',  '746',  '745',  '744',  '740'
)
ORDER BY  `id` DESC 
LIMIT 3

Answer 1

试试这个:

SELECT A.*, u.name 
FROM (SELECT c.*, IF(@updateId = @updateId:=update_id, @idx:=@idx+1, @idx:=0) idx 
      FROM comms c, (SELECT @updateId:=0, @idx:=0) A
      WHERE update_id IN ('1451', '1416', '1186', '1157', '1150', '1122', '1057', '914', '850', '816', '794', '790', '749', '746', '745', '744', '740')
      ORDER BY update_id, id DESC   
    ) AS A 
INNER JOIN users AS u ON u.id = A.user_id
WHERE A.idx < 3