处理sql 自连接 的问题 distinct 'null'

1.先看一下这段代码：
     select car.vin from car_info_temp car join (
        select brand from car_info_temp car where car.dealer_code=#{code} group by brand
    ) car1 on car.brand=car1.brand
 
我的疑问是  vin   brand 是来自一个表 为什么不一次查？
select Distinct brand,vin from car_info_temp car where car.dealer_code=#{code}  

这样一次查，查询是不一样的
上面自连接   通过dealer_code查到品牌brand   再查到brand下面所有的vin
一次查      通过dealer_code查到品牌brand   再查到brand下面dealer_code为给定code的vin
 
2.
select distinct 'null' as level ,series as series from car_info_temp car join (
    select brand from car_info_temp car where car.dealer_code=#{code} group by brand
) car1 on car.brand=car1.brand

distinct 'null'这里是把返回的多列的null值处理成一条