MySQL分组排序取前N条记录以及生成自动数字序列--group by 后 limit 外加 rownumber
同事提了一个需求，要求按照某列分组，然后将各组的前几条抽取出来。
表结构
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[email protected]
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。
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;
Query OK, 0 rows affected (0.00 sec)
Query OK, 0 rows affected (0.00 sec)
+------------+---------------+--------+------------+-----------+--------+
| time | machine | module | total_flow | deny_flow | rownum |
+------------+---------------+--------+------------+-----------+--------+
| 1409640360 | 10.201.20.181 | all | 53937 | 6058 | 1 |
| 1409640300 | 10.201.20.181 | all | 52588 | 5701 | 2 |
| 1409640240 | 10.201.20.181 | all | 54254 | 5608 | 3 |
| 1409640180 | 10.201.20.181 | all | 54684 | 5811 | 4 |
| 1409640360 | 10.201.20.97 | all | 50679 | 5307 | 1 |
| 1409640300 | 10.201.20.97 | all | 50472 | 5239 | 2 |
| 1409640240 | 10.201.20.97 | all | 51586 | 5509 | 3 |
| 1409640180 | 10.201.20.97 | all | 50794 | 5378 | 4 |
| 1409640360 | 10.201.20.98 | all | 84747 | 5652 | 1 |
| 1409640300 | 10.201.20.98 | all | 84506 | 5696 | 2 |
| 1409640240 | 10.201.20.98 | all | 84982 | 5513 | 3 |
| 1409640180 | 10.201.20.98 | all | 83997 | 5623 | 4 |
+------------+---------------+--------+------------+-----------+--------+有同学推荐了这个链接： http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/