Question

I have a Customers table with a columns called CustomerID, CustomerNumber. I have an Orders table that references the CustomerID table in Customers.

I need to find and remove all duplicate records for CustomerNumber that DO NOT have orders in the Orders table.

This is how I am finding my duplicates in the Customers but I do not know how to filter out by the orders table then remove the extra records:

SELECT Name, CustomerNumber, COUNT(*) As DupeCount
FROM StagingCustomers 
WHERE ManufacturerID=15
GROUP BY Name, CustomerNumber
HAVING COUNT(CustomerNumber) > 1
ORDER BY CustomerNumber

Answer 1

Add an EXISTS clause to check for any records without a relation:

SELECT Name, CustomerNumber, COUNT(*) As DupeCount
FROM StagingCustomers S
WHERE ManufacturerID=15
AND NOT EXISTS (SELECT 1 from Orders WHERE CustomerID = S.CustomerID)
GROUP BY Name, CustomerNumber
HAVING COUNT(CustomerNumber) > 1
ORDER BY CustomerNumber

EDIT:

Example of how to delete records below. This will set IsDeleted to 1 for every record that meets the above criteria, AND is not the lowest customerid per CustomerNumber. For a clearer answer you need to give your table layout and relations.

UPDATE  S
SET IsDeleted = 1
FROM StagingCustomers S
WHERE ManufacturerID=15
AND NOT EXISTS (SELECT 1 from Orders WHERE CustomerID = S.CustomerID)
AND CustomerID NOT IN (SELECT CustomerNumber, MIN(CustomerID)
                       FROM StagingCustomers
                       GROUP BY CustomerNumber)
GROUP BY Name, CustomerNumber
HAVING COUNT(CustomerNumber) > 1
ORDER BY CustomerNumber