如何在mysql上添加條件以基於列值在不同表上獲取值

Question

美好的一天，

我有4個要加入的表格。 現在我想根據交易類型獲取每個交易的詳細信息。 如果

交易表

id |     type    |  source_id
 1   'product'         1
 2   'reservation'     1
 3   'service'         1 

產品表

 id | Name     
  1 | product 1     

預約表

 id | reservation name        | other details
  1 |  some reservation name  | ------

服務表

 id | Service Name            | Other details
  1 | House Cleaning Service  | ------------

這是我想要的桌子

id |     type    |  source_id  | product_name  | reservation_name | service name
 1   'product'         1         product 1           null              null
 2   'reservation'     1             null      some reservation name   null
 3   'service'         1            null             null              House Cleaning Service

目前我最瘋狂的猜測是

select a.id,a.type,a.source_id,b.product_name,c.reservation_name,d.service_name
from transaction as a
left join product_table as b on a.source_id = b.id
left join reservation_table as c on a.source_id = c.id
left join services_table as d on a.source_id = d.id

而且我確定它不會100％：D

有沒有一種方法可以添加條件，如果'type'列='product'，那么它只能從products表中提取？

Answer 1

您可以嘗試使用CASE指令，如下所示：

select a.id,a.type,a.source_id,
       CASE a.source_id
           WHEN 'product'  THEN b.product_name
           else  null
       END ,
       CASE a.source_id
           WHEN 'reservation'  THEN c.reservation_name
           else  null
       END ,
       CASE a.source_id
           WHEN 'service'  THEN d.service_name
           else  null
       END 
from transaction as a
left join product_table as b on a.source_id = b.id
left join reservation_table as c on a.source_id = c.id
left join services_table as d on a.source_id = d.id

Answer 2

在連接中添加條件，例如

select a.id,a.type,a.source_id,b.product_name,c.reservation_name,d.service_name
from transaction as a
left join product_table as b on a.source_id = b.id and a.type='product'
left join reservation_table as c on a.source_id = c.id and a.type='reservation'
left join services_table as d on a.source_id = d.id and a.type='service'

Answer 3

select  a.id,a.type,a.source_id,b.product_name,c.reservation_name,d.service_name
from transaction as a
left join product_table as b on a.source_id = b.id
left join reservation_table as c on a.source_id = c.id
left join services_table as d on a.source_id = d.id WHERE a.type = 'product' 

嘗試這一希望對您有幫助