sql - 左连接列上的可选过滤器

Question

我在围绕如何编写在左连接上包含可选过滤器的过程时遇到问题。

我有两个表，问题和 Customer_Location。并非所有问题都与客户位置相关联。所以我认为这是一个起点:

SELECT
  I.Issue_Number,
  C.Customer_Location_Code

FROM Issues I
LEFT JOIN Customer_Location C
  ON C.Customer_Location_Key = I.Customer_Location_Key


Issue_Number | Customer_Location_Code
1            | Chicago
2            | NULL
3            | Chicago
4            | New York      

这有效，它给了我所有的问题。但我想为客户位置代码添​​加一个可选参数，如果留空将返回所有 4 个问题，但如果为芝加哥设置为 1，则仅返回问题 1 和 3。

我试过这个

DECLARE @customer_location_key INT
SET @customer_location_key = 1

SELECT
  I.Issue_Number,
  C.Customer_Location_Code

FROM Issues I
LEFT JOIN Customer_Location C
  ON C.Customer_Location_Key = I.Customer_Location_Key
 AND C.Customer_Location_Key = @customer_location_key

但我得到以下结果

Issue_Number | Customer_Location_Code
1            | Chicago
2            | NULL
3            | Chicago
4            | NULL

出于某种原因，我现在似乎在放屁，但似乎无法理解 应该 做一些比较简单的事情

Answer 1

添加一个类似于下面的 where 子句应该可以做到。

DECLARE @customer_location_key INT
SET @customer_location_key = 1

SELECT
  I.Issue_Number,
  C.Customer_Location_Code

FROM Issues I
LEFT JOIN Customer_Location C
  ON C.Customer_Location_Key = I.Customer_Location_Key
where (@customer_location_key is null or C.Customer_Location_Key = @customer_location_key)