Question

我有一个包含6列的user_accounts表：

id, user_id, amount, type, total_balance, created_at
1   101        10    debit       90       2018-09-14 20:10:49
2   101        30    credit     120       2018-09-14 20:10:52
3   102       210    credit     310       2018-09-14 21:10:52
4   102        10    debit      300       2018-09-14 21:10:54
5   103        10    credit     110       2018-09-14 21:10:54 
6   104        15    credit     115       2018-09-14 21:11:59

我想保留每个用户的最新n行，并删除其余行。

在mysql中构造此查询的最佳方法是什么？

Answer 1

您可以尝试在子查询中按每个userId设置行号，并按created_at排序。

然后通过ID删除每个userId上的rn

根据我的示例，我只保留第一行，所以我设置了rn > 1

CREATE TABLE T(
  id  int,
  user_id int,
  amount int, 
  type varchar(50), 
  total_balance int, 
  created_at datetime
);



insert into T values (1,101,10 ,'debit',90 ,'2018-09-14 20:10:49');
insert into T values (2,101,30 ,'credit',120,'2018-09-14 20:10:52');
insert into T values (7,101,30 ,'credit',120,'2018-09-16 20:10:52');
insert into T values (3,102,210,'credit',310,'2018-09-14 21:10:52');
insert into T values (4,102,10 ,'debit',300,'2018-09-14 21:10:54');
insert into T values (5,103,10 ,'credit',110,'2018-09-14 21:10:54'); 
insert into T values (6,104,15 ,'credit',115,'2018-09-14 21:11:59');


DELETE FROM T
WHERE ID IN (
    SELECT ID 
    FROM 
    (
       SELECT ID,(
        SELECT COUNT(*) 
        FROM T tt
        WHERE
          tt.user_id = t1.user_id 
        AND 
          tt.created_at >= t1.created_at
        ORDER BY 
           tt.created_at desc
      ) rn
      FROM T t1
    ) deltable
    where rn > 1 # n least rows you want to keep.
);

查询1 ：

SELECT * FROM T

Results ：

| id | user_id | amount |   type | total_balance |           created_at |
|----|---------|--------|--------|---------------|----------------------|
|  7 |     101 |     30 | credit |           120 | 2018-09-16T20:10:52Z |
|  4 |     102 |     10 |  debit |           300 | 2018-09-14T21:10:54Z |
|  5 |     103 |     10 | credit |           110 | 2018-09-14T21:10:54Z |
|  6 |     104 |     15 | credit |           115 | 2018-09-14T21:11:59Z |

Answer 2

您可以尝试以下查询。这里N是您要保留的行数

DELETE FROM user_accounts WHERE id NOT IN (select id FROM user_accounts ORDER BY created_at DESC LIMIT N)

Answer 3

这将适用于记录最近的3条记录：

create table ns_t(t timestamp);
desc ns_t;

insert into  ns_t values(current_timestamp);
insert into  ns_t values(current_timestamp + INTERVAL '2' HOUR);
insert into  ns_t values(current_timestamp + INTERVAL '4' HOUR);
insert into  ns_t values(current_timestamp + INTERVAL '8' HOUR);
insert into  ns_t values(current_timestamp + INTERVAL '10' HOUR);
insert into  ns_t values(current_timestamp + INTERVAL '12' HOUR);
select * from (select * from ns_t order by t desc) where rownum<=3;

如何保留每个用户的最新N行并删除mysql中的其余行？

3 个答案: