SQL - 选择相同的列两次

Question

我不知道它是否正确，但请查看以下架构：

实体可以是承运人或客户。每个客户都必须与承运人相关联，该承运人可以是自己的客户。

我遇到困难，尝试没有成功执行单个查询，该查询返回客户的 NAME 以及其运营商的 NAME 。有没有办法进行这样的操作？

由于

Answer 1

我将id_carrier解释为运营商的实体ID：

select ec.name as CustomerName, eca.name as CarrierName
from customer c join
     entity ec
     on c.id_entity = e.id_entity join
     entity eca
     on c.id_carrier = eca.id_entity

这消除了加入运营商表的需要，除非您需要其他信息。

Answer 2

您需要针对entity表加入两次，每种类型一次。使用他们的别名（下面的ca, cu）来引用SELECT列表中的每个。

SELECT
  /* Carrier info */
  ca.id_entity AS carrier_id_entity,
  ca.name AS carrier_name,
  ca.date AS carrier_date,
  carrier.carrier_stuff,
  /* Customer info */
  cu.id_entity_as AS customer_id_entity,
  cu.name AS customer_name,
  cu.date AS customer_date,
  customer.discount,
  customer.payment
FROM
  carrier 
  JOIN customer ON customer.carrier_id =  carrier.id_entity
  /* JOIN first against entity for carrier */
  JOIN entity ca ON carrier.id_entity = ca.id_entity
  /* and again between customer and entity */
  JOIN entity cu ON customer.id_entity = cu.id_entity

Answer 3

加入entity两次并提供name列别名。如果不需要其他carrier字段，请删除最后一个联接。

select customer_entity.name as customer_name, customer.credit, -- additional customer fields...
        carrier_entity.name as carrier_name, carrier.carrier_stuff -- additional carrier fields...
        from entity as customer_entity
        inner join customer on (customer.id_entity = customer_entity.id_entity)
        inner join entity as carrier_entity on (customer.id_carrier = carrier_entity.id_entity)
        inner join carrier on (carrier.id_entity = carrier_entity.id_entity)