mysql每组前N条

SELECT * from (
select
  case when t.province=@prov then @rownum:=@rownum+1
  else @rownum:=1 end rownum,
  case when t.province=@prov then 0
  else @prov:=t.province end prov,

  t.*
from
  (

    SELECT '' province,1231 money
    UNION all
    SELECT '' province,123121 money
    UNION all
    SELECT '' province,1231231 money
    UNION all
    SELECT '' province,121231 money
    UNION all
    SELECT '' province,123121 money
    UNION all
    SELECT '' province,12131 money
    UNION all
    SELECT '' province,12231 money
    UNION all
    SELECT '' province,123311 money

  ) t,(select @rownum:=0) rn,(select @prov:='') prov
ORDER BY t.province,t.money desc
) tt
where tt.rownum<=2
;