带有Case语句的SQL查询

Question

我想检查客户是否有特定产品。如果他们这样做，请返回是。如果他们不这样做，请返回否。

所以，我写了一个返回它的查询。但是，我不想输入ProductFamily名称。我想要加入。

SELECT 
    ID,
    (CASE 
        WHEN ProductFamily IN ('Product')
           THEN 'Yes'
           ELSE 'No' 
     END) AS 'O'
FROM
    [source].[Opportunity] A 

以下查询让我知道ProductFamily映射：

SELECT *
FROM [reference].[ProductFamilyMapping]
WHERE (isA = 1)
ORDER BY productfamilyname

你如何加入案例陈述？

Answer 1

您可以将子查询放在IN语句而不是硬编码文本中，但您需要指定列名称（即“SELECT mycolumn”而不是“SELECT *”）。

SELECT 
    ID
    ,(CASE 
        WHEN  ProductFamily IN 
        (SELECT ProductFamily FROM [reference].[ProductFamilyMapping] where (isA = 1))
          THEN 'Yes'
        ELSE 'No' END) as 'O'
From  [source].[Opportunity] A 

（我假设[reference]中的列名。[ProductFamilyMapping]也是ProductFamily）。

Answer 2

另一种方法是使用CTE和LEFT JOIN。试试这个：

WITH PFM AS (
  SELECT DISTINCT ProductFamily FROM [reference].[ProductFamilyMapping] where (isA = 1)
)
SELECT 
    ID
    ,CASE WHEN PFM.ProductFamily IS NULL THEN 'No' ELSE 'Yes' END AS 'O'
From  [source].[Opportunity] A 
LEFT JOIN PFM ON A.ProductFamily = PFM.ProductFamily