如何指定何时连接到具有相同主键名称的表？

  • Z时代
  • 2024-01-10
  • 分类：问答

select request_id, request_created_date, request_complaint_type, request_descriptor, 
request_ferry_terminal 
from nyc311_requests 
    join nyc311_locations_ferries on request_id=request_id 
where request_ferry_direction = 'Manhattan Bound' 
order by request_id

我有这段代码，当我尝试执行时，它说每次提到request_id时，它都是一个不明确的列。如何指定何时连接到具有相同主键名称的表？

回答：

你需要看到该表是从的，请注意我已经添加了表名到REQUEST_ID

select nyc311_requests.request_id, request_created_date, request_complaint_type, request_descriptor, 
request_ferry_terminal 
from nyc311_requests 
    join nyc311_locations_ferries on nyc311_requests.request_id=nyc311_locations_ferries.request_id 
where request_ferry_direction = 'Manhattan Bound' 
order by request_id

，你也可以给表的别名

select r.request_id, request_created_date, request_complaint_type, request_descriptor, 
request_ferry_terminal 
from nyc311_requests r 
    join nyc311_locations_ferries lf on r.request_id=lf.request_id 
where request_ferry_direction = 'Manhattan Bound' 
order by request_id

回答：

你必须给你的表别名。

select A.request_id, request_created_date, request_complaint_type, request_descriptor, 
request_ferry_terminal 
from nyc311_requests A 
    join nyc311_locations_ferries B on A.request_id = B.request_id 
where request_ferry_direction = 'Manhattan Bound' 
order by request_id

你必须指定出现两个表中的字段中的别名，但一旦你知道如何做到这一点是非常简单的。

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