Mysql中实现row_number

CREATE TABLE `zsl_test` (  
        `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 zsl_test values (1,1,80);  
    insert into zsl_test values (2,1,80);  
    insert into zsl_test values (3,1,80);  
    insert into zsl_test values (4,1,80);  
    insert into zsl_test values (5,2,80);  
    insert into zsl_test values (6,2,80);  
    insert into zsl_test values (7,2,80);  
    insert into zsl_test values (8,2,80);  
    insert into zsl_test values (9,2,80);  
    insert into zsl_test values (10,2,80);  

    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 zsl_test order by id   
    ) b ,(select @rownum :=0 , @pdept := null ,@rank:=0) c ) result  
    having rank <3 ;