MySQL 实现row_number() 分组排序功能

数据库基本脚本

CREATE TABLE `a` (
	`ID` INT(10) NULL DEFAULT NULL,
	`class` INT(10) NULL DEFAULT NULL,
	`score` INT(10) NULL DEFAULT NULL
)
COLLATE='utf8_general_ci'
ENGINE=InnoDB;

insert into a values (1,1,110);
insert into a values (2,1,120);
insert into a values (3,1,130);
insert into a values (4,1,140);
insert into a values (5,2,210);
insert into a values (6,2,220);
insert into a values (7,2,230);
insert into a values (8,2,240);
insert into a values (9,3,310);
insert into a values (10,3,320);
insert into a values (11,4,410);

　1.　对 class进行分组 取每组的前2条

select id,class,score,rank from (
select b.id,b.class,b.score,@rownum:=@rownum+1 ,
if(@pdept=b.class,@rank:=@rank+1,@rank:=1) as rank,
@pdept:=b.class
from (
select id,class,score from a order by id 
) b ,(select @rownum :=0 , @pdept := null ,@rank:=0) c ) result
having rank <3 ;

　rank <3 指的相当月ruwnum<3 每组都自动给上了序号.多少条记录就多少个序号.指的是每组的.

 2.  对 class 进行分组   取记录满足两天的,也就是rownum=2 的数据

select id,class,score,rank from (
select b.id,b.class,b.score,@rownum:=@rownum+1 ,
if(@pdept=b.class,@rank:=@rank+1,@rank:=1) as rank,
@pdept:=b.class
from (
select id,class,score from a order by score desc 
) b ,(select @rownum :=0 , @pdept := null ,@rank:=0) c ) result
having rank =2 ;