mysql实现rownum-白红宇

mysql实现rownum

阅读量：5142 次

发布时间：2019-06-13

本文共 4429 字，大约阅读时间需要 14 分钟。

CREATE

TABLE `total_freq_ctrl` (

`time` int(10) unsigned NOT NULL,

`machine` char(64) NOT NULL,

`module` char(32) NOT NULL,

`total_flow` int(10) unsigned NOT NULL,

`deny_flow` int(10) unsigned NOT NULL,

PRIMARY KEY (`module`,`machine`,`time`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8

原sql

SELECT machine, deny_flow, total_flow, time FROM total_freq_ctrl A WHERE 1 > (SELECT COUNT(machine) FROM total_freq_ctrl WHERE machine = A.machine AND time > A.time) AND A.module = 'all' ORDER BY A.time desc;

只要将 1 改成N就变成取每组的前N条，因为我极其不喜欢子查询，就改就尝试改称join 的方式。

不过这里需要对所有数据进行排序才能确定每组的前N条，所以最佳优化也要全表扫描一次。

首先我要对表中数据进行排序，引入一个变量@row来做rownumber

set @row=0;set @mid='';SELECT module, machine, time, @row:=@row+1 rownum FROM total_freq_ctrl order by module,machine,time desc limit 10;

Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

+--------+---------------+------------+--------+

| module | machine | time | rownum |

+--------+---------------+------------+--------+

| all | 10.201.20.181 | 1409640060 | 1 |

| all | 10.201.20.181 | 1409640000 | 2 |

| all | 10.201.20.181 | 1409639940 | 3 |

| all | 10.201.20.181 | 1409639880 | 4 |

| all | 10.201.20.97 | 1409640060 | 5 |

| all | 10.201.20.97 | 1409640000 | 6 |

| all | 10.201.20.97 | 1409639940 | 7 |

| all | 10.201.20.97 | 1409639880 | 8 |

| all | 10.201.20.98 | 1409640060 | 9 |

| all | 10.201.20.98 | 1409640000 | 10 |

+--------+---------------+------------+--------+

rownumber已经出来了，再加一个@mid来进行分组

set @row=0;set @mid='';SELECT module, machine, time,case when @mid = machine then @row:=@row+1 else @row:=1 end rownum, @mid:=machine FROM total_freq_ctrl order by module,machine,time desc limit 20;

Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

+--------+---------------+------------+--------+---------------+

| module | machine | time | rownum | @mid:=machine |

+--------+---------------+------------+--------+---------------+

| all | 10.201.20.181 | 1409640180 | 1 | 10.201.20.181 |

| all | 10.201.20.181 | 1409640120 | 2 | 10.201.20.181 |

| all | 10.201.20.181 | 1409640060 | 3 | 10.201.20.181 |

| all | 10.201.20.181 | 1409640000 | 4 | 10.201.20.181 |

| all | 10.201.20.181 | 1409639940 | 5 | 10.201.20.181 |

| all | 10.201.20.181 | 1409639880 | 6 | 10.201.20.181 |

| all | 10.201.20.97 | 1409640180 | 1 | 10.201.20.97 |

| all | 10.201.20.97 | 1409640120 | 2 | 10.201.20.97 |

| all | 10.201.20.97 | 1409640060 | 3 | 10.201.20.97 |

| all | 10.201.20.97 | 1409640000 | 4 | 10.201.20.97 |

| all | 10.201.20.97 | 1409639940 | 5 | 10.201.20.97 |

| all | 10.201.20.97 | 1409639880 | 6 | 10.201.20.97 |

| all | 10.201.20.98 | 1409640180 | 1 | 10.201.20.98 |

| all | 10.201.20.98 | 1409640120 | 2 | 10.201.20.98 |

| all | 10.201.20.98 | 1409640060 | 3 | 10.201.20.98 |

| all | 10.201.20.98 | 1409640000 | 4 | 10.201.20.98 |

| all | 10.201.20.98 | 1409639940 | 5 | 10.201.20.98 |

| all | 10.201.20.98 | 1409639880 | 6 | 10.201.20.98 |

+--------+---------------+------------+--------+---------------+

好了，再外面加一层inner join 再对 rownumber 做限制就可以拿到目标数据了。

1. set @row=0;set @mid='';select a.*,b.rownum from total_freq_ctrl a inner join (SELECT module, machine, time, case when @mid = machine then @row:=@row+1 else @row:=1 end rownum, @mid:=machine mid FROM total_freq_ctrl order by module,machine,time desc) b on b.module=a.module and b.machine=a.machine and b.time=a.time where b.rownum<5;
2. Query OK, 0 rows affected (0.00 sec)
4. Query OK, 0 rows affected (0.00 sec)
6. +------------+---------------+--------+------------+-----------+--------+
7. | time | machine | module | total_flow | deny_flow | rownum |
8. +------------+---------------+--------+------------+-----------+--------+
9. | 1409640360 | 10.201.20.181 | all | 53937 | 6058 | 1 |
10. | 1409640300 | 10.201.20.181 | all | 52588 | 5701 | 2 |
11. | 1409640240 | 10.201.20.181 | all | 54254 | 5608 | 3 |
12. | 1409640180 | 10.201.20.181 | all | 54684 | 5811 | 4 |
13. | 1409640360 | 10.201.20.97 | all | 50679 | 5307 | 1 |
14. | 1409640300 | 10.201.20.97 | all | 50472 | 5239 | 2 |
15. | 1409640240 | 10.201.20.97 | all | 51586 | 5509 | 3 |
16. | 1409640180 | 10.201.20.97 | all | 50794 | 5378 | 4 |
17. | 1409640360 | 10.201.20.98 | all | 84747 | 5652 | 1 |
18. | 1409640300 | 10.201.20.98 | all | 84506 | 5696 | 2 |
19. | 1409640240 | 10.201.20.98 | all | 84982 | 5513 | 3 |
20. | 1409640180 | 10.201.20.98 | all | 83997 | 5623 | 4 |
21. +------------+---------------+--------+------------+-----------+--------+