MySQL groups and sorts the first N records and generates an automatic number sequence. -- Group by, limit, and rownumber_MySQL

My colleague asked me to group by column and then extract the first few items from each group. Table structure CREATETABLE 'total _ freq_ctrl '('time' int (10) unsignedNOTNULL, 'Machine' char (64) NOTNULL, then extract the first few entries of each group.

Table structure

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

Original 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;

Change 1 to N to get the first N records of each group. because I do not like subqueries very much, I will try to rename the join method.

However, we need to sort all the data to determine the first N entries in each group. Therefore, we need to perform a full table scan for the best optimization.

First, I want to sort the data in the table and introduce a variable @ row for 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 already exists. add @ mid to group

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  |+--------+---------------+------------+--------+---------------+

Now, add an inner join to the table and then limit rownumber to get the target data.

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 |+------------+---------------+--------+------------+-----------+--------+