SQL - 查询以报告 MAX DATE 结果或 NULL 结果

Question

我有一个包含 3 个表的 SQL 数据库：

1. Customer record 表，保存主数据（每一行都是唯一的）
2. Customer notes，每个笔记数据/时间戳（每个客户可能有很多笔记，或者根本没有）

3. Product 表格，显示哪个客户购买了哪个产品

   表：tbl_Customer_Records
   CustomerID---- 公司名称----- 公司段----- 促销代码

   表格：tbl_Customer_Notes
   CustomerID---- 注意-----创建日期

   表：tbl_Customer_Products
   CustomerID---- 产品类别

我想要的是提取仅包含最新注释的客户记录列表，因此如果存在多个注释，则不会出现重复行。但如果没有备注根本不存在，我也希望我的报告包含客户记录。我已经使用 SELECT MAX 函数实现了第一部分，并且效果很好，问题是当我在下面的最后一行代码中添加 OR = NULL 子句时。这不起作用，我想不出解决办法。

任何建议将不胜感激！

SELECT        

[tbl_Customer_Records].[CustomerID], 
[tbl_Customer_Records].[Company Name], 
[tbl_Customer_Records].[Company Segment], 
[tbl_Customer_Records].[Promo Code], 
[tbl_Customer_Notes].[Note],
[tbl_Customer_Products].[Product Category]

FROM            

tbl_Customer_Records
LEFT OUTER JOIN tbl_Customer_Notes
ON tbl_Customer_Records.CustomerID = tbl_Customer_Notes.CustomerID 
LEFT OUTER JOIN tbl_Customer_Products
ON tbl_Customer_Records.CustomerID = tbl_Customer_Products.CustomerID

WHERE  
[Product Category] in ('Nuts','Bolts','Screws','Spanners') 

AND 

[Created Date] in (SELECT MAX ([Created Date]) FROM tbl.Customer_Notes GROUP BY [CustomerID]) 

OR tbl_Customer_Note.Note is null

Answer 1

有一些技巧可以进行这种查询（row_number 或加入分组数据），但我认为在您的情况下最简洁的方法是使用外部应用：

select
   cr.[CustomerID], 
   cr.[Company Name], 
   cr.[Company Segment], 
   cr.[Promo Code], 
   cn.[Note],
   cp.[Product Category]
from tbl_Customer_Records as cr
    left outer join tbl_Customer_Products as cp on cp.CustomerID = cr.CustomerID
    outer apply (
        select top 1
            t.[Note]
        from tbl_Customer_Notes as t
        where t.[CustomerID] = cr.[CustomerID]
        order by t.[Created_Date] desc
    ) as cn
where
    cp.[Product Category] in ('Nuts','Bolts','Screws','Spanners') 

将所有笨拙的table name.column name 更改为alias.column name，我认为这样更易读。

或者：

select
    cr.[CustomerID], 
    cr.[Company Name], 
    cr.[Company Segment], 
    cr.[Promo Code], 
    cn.[Note],
    cp.[Product Category]
from tbl_Customer_Records as cr
    left outer join tbl_Customer_Products as cp on cp.CustomerID = cr.CustomerID
    left outer join tbl_Customer_Notes as cn on
        cn.CustomerID = cr.CustomerID and
        cn.[Created_Date] = (select max(t.[Created_Date]) from tbl_Customer_Notes as t where t.CustomerID = cr.CustomerID)
where
    cp.[Product Category] in ('Nuts','Bolts','Screws','Spanners')

Answer 2

您可以在ON 谓词中添加过滤条件，以保留左表中的行，并从第一个 LEFT OUTER JOIN 运算符中仅从右表中获取所需的匹配行。以下查询应该有效：

SELECT        
CR.[CustomerID], 
CR.[Company_Name], 
CR.[Company_Segment], 
CR.[Promo_Code], 
CN.[Note],
CP.[Product_Category]
FROM            
tbl_Customer_Records CR
LEFT OUTER JOIN tbl_Customer_Notes CN
ON CR.CustomerID = CN.CustomerID AND CN.[Created_Date] in (SELECT MAX ([Created_Date]) 
                                                           FROM tbl_Customer_Notes 
                                                           WHERE CR.CustomerID =  tbl_Customer_Notes.CustomerID 
                                                           GROUP BY [CustomerID]) 
LEFT OUTER JOIN tbl_Customer_Products CP
ON CR.CustomerID = CP.CustomerID
WHERE  
[Product_Category] in ('Nuts','Bolts','Screws','Spanners') 

Answer 3

应该可以，尝试使用 NULL 值：

SELECT a.[CustomerID], 
       a.[Company Name], 
       a.[Company Segment], 
       a.[Promo Code], 
       a.[Note],
       a.[Product Category]
FROM (
SELECT        
cr.[CustomerID], 
cr.[Company Name], 
cr.[Company Segment], 
cr.[Promo Code], 
cn.[Note],
cp.[Product Category],
ROW_NUMBER() OVER(PARTITION BY cr.[CustomerID] ORDER BY cn.[Created Date] DESC) as rnk
FROM tbl_Customer_Records cr
LEFT JOIN tbl_Customer_Notes cn
ON cr.CustomerID = cn.CustomerID 
LEFT JOIN tbl_Customer_Products cp
ON cr.CustomerID = cp.CustomerID
WHERE cp.[Product Category] in ('Nuts','Bolts','Screws','Spanners') )a
WHERE a.rnk = 1

Answer 4

尝试使用CustomerID not in (select CustomerID from tbl_Customer_Note)