在MySQL平台中加入表只有几行[重复]

Question

我在加入表格时遇到问题。我已经在谷歌搜索但无法解决这个问题。我想在连接表中获得最多 4 行。我把我的虚拟表结构放在这里。

here is table `users`

--------------------
| id | name        |
--------------------
| 1  | John        |
--------------------
| 2  | Mohn        |
--------------------

here is table `user_transections`

------------------------------------------------
| id | user_id | amount  |    created_at       |
------------------------------------------------
| 1  | 1       | 20      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------
| 2  | 1       | 30      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------
| 3  | 1       | 50      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------
| 4  | 1       | 60      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------
| 5  | 2       | 10      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------
| 6  | 2       | 15      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------
| 7  | 2       | 80      | xxxx-xx-xx xx:xx:xx |
------------------------------------------------

我只想为每个用户表匹配加入 3 行 SELECT user.name,user_transections.amount FROM users INNER JOIN user_transections on user.id = user_transections.user_id // 如何在此处添加限制以加入最多三行横断面表

Answer 1

这对 MySQL 来说是个痛点。最通用的方式是使用变量：

select ut.*
from (select ut.*,
             (@rn := if(@u = user_id, @rn + 1,
                        if(@u := user_id, 1, 1)
                       )
             ) as rn
      from (select ut.*
            from user_transections ut
            order by user_id, created_at desc
           ) ut cross join
           (select @u := -1, @rn := 0) params
     ) ut
where rn <= 3;

如果您只有两个用户（这似乎不太可能），union all 更简单：

(select ut.*
 from user_transactions ut
 where user_id = 1
 order by created_at desc
 limit 3
) union all
(select ut.*
 from user_transactions ut
 where user_id = 2
 order by created_at desc
 limit 3
);

第三种方法使用相关子查询。这是一个版本：

select ut.*
from user_transactions ut
where ut.id >= coalesce( (select ut2.id
                          from user_transactions ut2
                          where ut2.user_id = ut.user_id
                          order by ut2.id desc
                          limit 1 offset 2
                         ), ut.id
                       );

为了获得此查询的性能，您需要user_transactions(user_id, id) 上的索引。