如何样才能合并sql并按b表的字段排序

怎么样才能合并sql并按b表的字段排序啊
a表sql：select a.id,a.userid,a.username,a.shangjianame
from zu_car_old_shangjia a

b表sql:select count(id) as carnum,userid from zu_car_old
  where passok=1
  group by userid

现在我想把上面sql合成一句,以a表为主，按b表查询出来的carnum从大到小排，两表的userid是相等的,即按发布车的数量，把商家排出来？

[解决办法]

SQL code

select a.id,a.userid,a.username,a.shangjianame,b.carnumfrom zu_car_old_shangjia ajoin(select count(id) as carnum,userid from zu_car_old  where passok=1  group by userid) bon a.userid=b.useridorder by b.carnum desc