如果三个表都是右外连接，怎么区分哪个表是右表呢？
select
t1.merchant_no,
t2.province,
t3.city,
t4.area
from
zl_customer_base t1
right join zl_provinces t2 on
t1.reg_address_one_level = t2.provinceid
right join zl_cities t3 on
t1.reg_address_two_level = t3.cityid
right join zl_areas t4 on
t1.reg_address_four_level = t4.areaid
