内部连接到同一个表两次（ALIASES）

Question

我的内部联接存在问题，这些是FK和PK

的表格

TABLE CITY
city_id (PK)
city_name
state

TABLE DEPOT
dep_id (PK)
capacity
city_id (FK) references CiTy

TABLE MANUFACTURER
manu_id (PK)
manu_name
city_id (FK) references city

所以我只想让结果看起来像这样：

DEPOT_CITY_name(references from city_id)，MANUFACTURER_CITY_name(references from city_id)

所以depot_city_name会显示name of city depo的{​​{1}}

和references CITY.city_id将显示manufacturer_city_name name of city的{​​{1}}

感谢

Answer 1

你需要一个左外连接：

select c.city_id, d.depo_id, m.manu_id,c.city_name
from city c
left outer join depo d on c.city_id=d.city_id
left outer join manufacturer m on c.city_id=m.city_id

编辑：如果您想将depo和制造商的城市名称分开，请使用：

select c.city_id, d.depo_id, m.manu_id,
case 
when  d.depo_id is null then null else c.city_name end DEPOT_CITY_name,
case 
when m.manu_id is null then null else c.city_name end manufacturer_city_name 
from city c
left outer join depo d on c.city_id=d.city_id
left outer join manufacturer m on c.city_id=m.city_id