MySQL 实现row_number() 分组排序功能

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);

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 ;

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 ;