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，--grouprownumber
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
tudou@b2c.xiaomi.com
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。
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 | +------------+---------------+--------+------------+-----------+--------+
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/
# mysql不支持其它复杂数据库的类似 rank() over 的排名和统计查询
# 只能通过变通的子查询和逻辑计算方式来实现，对于中小数据量可以考虑
-- rank 排名实现
select inline_rownum, aa, cc, amt, orderid FROM
(
select
# logic_cal 只是实现计数器计算的，每次逐条查询时会对比当前 cc 与 @last_cc 是否相同，如果不同则把当前该列值赋于 @last_cc 并重设计数器 @num := 1，否则计数器自加 @num := @num + 1
(case when cc <> @last_cc then concat(@last_cc := cc, @num := 1 ) else concat(@last_cc, @num := @num + 1) end ) logic_cal
, @num as inline_rownum
, aa, cc, amt, orderid
from tb_rank,
( select @last_cc := '') t, # 初始化 @last_cc 为 '', 如要检查的列(基于计数器统计的列)是int型，则初始化为0; varchar型初始化为''
( select @num := 0 ) t2 # 初始化@num为0
order by cc, orderid asc # 排序的方式会影响@num的生成，因为logic_cal是逐行计算的
) t
where inline_rownum <= floor(amt*0.8) #限制条数，取常量值或其他
order by cc,orderid asc
;
你这个Sql语句本身都有问题，按分组的话，你显示的字段都要放在分组里；
sql语句正确后，按时间降序排列是正确的
本站文章为和通数据库网友分享或者投稿，欢迎任何形式的转载，但请务必注明出处.
同时文章内容如有侵犯了您的权益，请联系QQ：970679559，我们会在尽快处理。