T-SQL JOIN Table On Self Based on Closest Date - sql

Question

Thank you in advance for reading!
The question I'm trying to answer is: "How much do parts really cost to make?" We manufacture by machining raw metal billets down to metal parts. Final parts are sold to a customer and scrap metal from the process is sold to the scrap yard.
For business/ERP configuration reasons our scrap vendor is listed as a customer and we ship him 'parts' like our other customers. These dummy parts are simply for each of the metal alloys we work with, so there is one dummy scrap part for each alloy we use. The scrap shipments are made whenever we fill our scrap bins so there's no defined time interval.
I'm trying to connect the ship date of a real part to a real customer to the closest scrap ship date of the same alloy. Then I can grab the scrap value per pound we were paid and include it in our revenue for the parts we make. If I can ask for the world it would be helpful to know how to grab the scrap shipment immediately before or immediately after the shipment of a real part - I'm sure management will change their minds several times debating if they want to use the 'before' or 'after' number.
I've tried other solutions and can't get them to work. I'm crying uncle, I simply can't get it to work....the web SQL interface our ERP uses claims it's T-SQL... thank you for reading this far!
What I'd like the output to look like is:
Customer Part Price Alloy Weight_Lost Scrap_Value Ship_Date
ABC Widget1 99.99 C182 63 2.45 10-01-2016
Here's the simplest I can boil the tables down to:
SELECT
tbl_Regular_Sales.Customer
tbl_Regular_Sales.Part
tbl_Regular_Sales.Price
tbl_Regular_Sales.Alloy
tbl_Regular_Sales.Weight_Lost
tbl_Scrap_Sales.Price AS 'Scrap_Value'
tbl_Regular_Sales.Ship_Date
FROM
(SELECT P.Part
,P.Alloy
,P.Price
,S.Ship_Date
,S.Customer
FROM Part AS P
JOIN S AS S
ON S.Part_Key = P.Part_Key
WHERE Shipper.Customer = 'Scrap_Yard'
) AS tbl_Scrap_Sales
JOIN
(SELECT P.Part
,P.Weight_Lost
,P.Alloy
,P.Price
,S.Ship_Date
,S.Customer
FROM Part AS P
JOIN S AS S
ON S.Part_Key = P.Part_Key
WHERE Shipper.Customer <> 'Scrap_Yard' ) AS tbl_Regular_Sales
ON
tbl_Regular_Sales.Alloy = tbl_Scrap_Sales.Alloy
AND <Some kind of date JOIN to get the closest scrap shipment value>

Answer 1

Something like this may do the trick:
WITH cteScrapSales AS (
SELECT
P.Alloy
,P.Price
,S.Ship_Date
FROM Part AS P
JOIN Shipper AS S ON S.Part_Key = P.Part_Key
WHERE S.Customer = 'Scrap_Yard'
), cteRegularSales AS (
SELECT
P.Part_Key
,P.Part
,P.Weight_Lost
,P.Alloy
,P.Price
,S.Ship_Date
,S.Customer
FROM Part AS P
JOIN Shipper AS S ON S.Part_Key = P.Part_Key
WHERE S.Customer <> 'Scrap_Yard'
)
SELECT
C.Customer
,C.Part
,C.Price
,C.Alloy
,C.Weight_Lost
,C.Scrap_Value
,C.Ship_Date
FROM (
SELECT R.*, S.Price AS Scrap_Value, ROW_NUMBER() OVER (PARTITION BY R.Part_Key ORDER BY DATEDIFF(SECOND, R.Ship_Date, S.Ship_Date)) ix
FROM cteRegularSales R
JOIN cteScrapSales S ON S.Allow = R.Allow AND S.Ship_Date > R.Ship_Date
) AS C
WHERE C.ix = 1;