SQL - query to report MAX DATE results or NULL results - sql

Question

I have an SQL database with 3 tables:
Customer record table, holding master data (each row is unique)
Customer notes, each note data/time stamped (there could be many notes per customer, or none at all)
Product table, showing which customer has bought which product
Table: tbl_Customer_Records
CustomerID---- Company Name-----Company Segment----- Promo Code
Table: tbl_Customer_Notes
CustomerID---- Note-----Created Date
Table: tbl_Customer_Products
CustomerID---- Product Category
What I want is to pull a list of customer records which includes the latest note only, so there are no duplicate lines if multiple notes exist. But I also want my report to include customer records if no notes exist at all. I've achieved the first part of this with a SELECT MAX function, and that works well, the problem is when I add the OR = NULL clause in the final line of code below. This doesn't work, and I can't figure out a solution.
Any suggestions will be greatly appreciated!
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

There're a few tricks to do this kind of query (row_number or join with grouped data), but I think most cleanest one in your case is to use outer apply:
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')
Changed all clumsy table name.column name to alias.column name, I think it's much more readable this way.
Or:
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

You can add your filter condition in ON predicate to preserve rows from left table and fetch only required matching rows from right table, from first LEFT OUTER JOIN operator. Following query should work:
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

Should work, tried with NULL values:
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

try to use CustomerID not in (select CustomerID from tbl_Customer_Note)