MySQL分組排序取前N條記錄以及產生自動數字序列--group by 後 limit 外加 rownumber，--grouprownumber

MySQL分組排序取前N條記錄以及產生自動數字序列--group by 後 limit 外加 rownumber，--grouprownumber

同事提了一個需求，要求按照某列分組，然後將各組的前幾條抽取出來。

表結構

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;

只要將 1 改成N就變成取每組的前N條，因為我極其不喜歡子查詢，就改就嘗試改稱join 的方式。

不過這裡需要對所有資料進行排序才能確定每組的前N條，所以最佳最佳化也要全表掃描一次。

首先我要對錶中資料進行排序，引入一個變數@row來做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已經出來了，再加一個@mid來進行分組

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，再外面加一層inner join 再對 rownumber 做限制 就可以拿到目標資料了。

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

有同學推薦了這個連結：http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/

mysql分組排序後取出幾條記錄，

# mysql不支援其它複雜資料庫的類似 rank() over 的排名和統計查詢
# 只能通過變通的子查詢和邏輯計算方式來實現，對於中小資料量可以考慮

-- rank 排名實現
select inline_rownum, aa, cc, amt, orderid FROM
(
select
# logic_cal 只是實現計數器計算的，每次逐條查詢時會對比當前 cc 與 @last_cc 是否相同，如果不同則把當前該列值賦於 @last_cc 並重設計數器 @num := 1，否則計數器自加 @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 為 '', 如要檢查的列(基於計數器統計的列)是int型，則初始化為0; varchar型初始化為''
( select @num := 0 ) t2 # 初始化@num為0
order by cc, orderid asc # 排序的方式會影響@num的產生，因為logic_cal是逐行計算的
) t
where inline_rownum <= floor(amt*0.8) #限制條數，取常量值或其他
order by cc,orderid asc
;
 
mysql 多表 查詢 之後 group by 分組 order by 不可以按照日期排序

你這個Sql語句本身都有問題，按分組的話，你顯示的欄位都要放在分組裡；
sql語句正確後，按時間降序排列是正確的