MySQL分组排序取前N条记录以及生成自动数字序列--groupby后limit外加rownumber_MySQL
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
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;
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 | +--------+---------------+------------+--------+
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 | +--------+---------------+------------+--------+---------------+
tudou@b2c.xiaomi.com
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 | +------------+---------------+--------+------------+-----------+--------+
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