mySql 实现排序函数row_number() over(partition by )

对b_id进行分组，并取每组的前两条结果，@rownum是申明局部变量rownum ,   :=是赋值符号，if(exp1,exp2,exp3)exp1为true，则返回exp2,否则返回exp3

select b_id,b_name,rank from (  

select b.b_id,b.b_name,@rownum:=@rownum+1 ,  
if(@id=b.b_id,@rank:=@rank+1,@rank:=1) as rank,  
@id:=b.b_id
from (  
select b_id,b_name from cky_book order by b_id   
) b ,(select @rownum :=0 , @id := null ,@rank:=0) c ) result  
having rank <3 ;