MS AdventureWorks dbo.ufnGetProductListPrice - sql

Question

I'm studing the Ms SQL AdventureWorks 2014, to model an internal dbase for our company
I usually work on Postgres, and I'm trying to "understand" Ms SQL stored procedures... :-)) BUT ....
The store Procedure [dbo.ufnGetProductListPrice ] SEEMS STRANGE to me.
the SQL code is found here:
(https://dataedo.com/samples/html/AdventureWorks/doc/AdventureWorks_2/functions/dbo_ufnGetProductListPrice_116.html)
CREATE FUNCTION [dbo].[ufnGetProductListPrice](#ProductID [int], #OrderDate [datetime])
RETURNS [money]
AS
BEGIN
DECLARE #ListPrice money;
SELECT #ListPrice = plph.[ListPrice]
FROM [Production].[Product] p
INNER JOIN [Production].[ProductListPriceHistory] plph
ON p.[ProductID] = plph.[ProductID]
AND p.[ProductID] = #ProductID
AND #OrderDate BETWEEN plph.[StartDate] AND COALESCE(plph.[EndDate], CONVERT(datetime, '99991231', 112)); -- Make sure we get all the prices!
RETURN #ListPrice;
END;
The function is making use of the following two tables:
[Production.Product] (https://dataedo.com/samples/html/AdventureWorks/doc/AdventureWorks_2/tables/Production_Product_153.html)
[Production.ProductListPriceHistory] - (https://dataedo.com/samples/html/AdventureWorks/doc/AdventureWorks_2/tables/Production_ProductListPriceHistory_159.html)
In particular my doubts are:
the function is getting the parameter #ProductID.
The same #ProductID is used as the primary key of the Production.Product AND as part of the primary key for the table Production.ProductListPriceHistory
so is seems of no help making a join on Product.[ProductID] = ProductListPriceHistory.[ProductID]
when we can test the ProductID directly on the ProductListPriceHistory.[ProductID]
Why to create such a join ? Seems of no help...
The given #Orderdate datetime received as second parameter, is checked in the JOIN against the following condition
AND #OrderDate BETWEEN plph.[StartDate] AND COALESCE(plph.[EndDate], CONVERT(datetime, '99991231', 112)); -- Make sure we get all the prices!
BUT, if we are calling the store procedure for #Orderdate = 1/1/2022,
considering that [EndDate] could be NULL,
and we could have in ProductListPriceHistory.StartDate two records with our #ProductID, the first with StartDate=1/1/2020 and the second StartDate=1/1/2021,
such "BETWEEN" condition should match both of them, when obviously we would expect the last one ....
is it a bug ?

Answer 1

You are right, there are a number of serious flaws in this code, and I would recommend finding better tutorials instead.
I've noted comments on each flaw
CREATE FUNCTION [dbo].[ufnGetProductListPrice](#ProductID [int], #OrderDate [datetime])
RETURNS [money] -- money is a bad data type due to rounding problems, decimal should be used
-- scalar UDFs are very slow, this should be a inline Table Valued Function
AS
BEGIN
DECLARE #ListPrice money;
SELECT #ListPrice = plph.[ListPrice] -- as you point out, there should be some kind of aggregation
FROM [Production].[Product] p -- as you point out: join is unnecessary
INNER JOIN [Production].[ProductListPriceHistory] plph
ON p.[ProductID] = plph.[ProductID]
AND p.[ProductID] = #ProductID
-- BETWEEN should never be used on dates, use ">= AND <"
AND #OrderDate BETWEEN plph.[StartDate] AND COALESCE(plph.[EndDate], CONVERT(datetime, '99991231', 112)); -- ISNULL is better for performance than COALESCE
RETURN #ListPrice;
END;
A better function would be this
CREATE FUNCTION dbo.ufnGetProductListPrice (
#ProductID int,
#OrderDate datetime
)
RETURNS TABLE
AS RETURN
SELECT TOP (1)
plph.ListPrice
FROM Production.ProductListPriceHistory plph
WHERE plph.ProductID = #ProductID
AND #OrderDate >= plph.StartDate
AND #OrderDate < ISNULL(plph.EndDate, CONVERT(datetime, '99991231', 112))
ORDER BY
plph.StartDate DESC;
Assuming it's possible for there to be multiple active prices (I don't think it's possible in AdventureWorks), then you also need TOP (1) and ORDER BY plph.StartDate DESC. If this is not possible then you can leave that out.
Instead of doing this with a scalar UDF
SELECT p.*, dbo.ufnGetProductListPrice(p.ProductId, GETDATE())
FROM Production.Product p;
You use an APPLY for a TVF
SELECT p.*, plp.*
FROM Production.Product p
OUTER APPLY dbo.ufnGetProductListPrice(p.ProductId, GETDATE()) plp;
An OUTER APPLY works like a LEFT JOIN, which means you get NULL if there are no rows.

Answer 2

#Charlieface
Thanks for your reply. One more note on the function ufnGetProductStandardCost
AdventureWorks is storing historical Retailprices and CostPrices, to let you found the price (list, cost) of a certain product at a certain time. This is why I'm interested in this part of the database
It even offers a "dual" function for Cost prices ufnGetProductStandardCost
https://dataedo.com/samples/html/AdventureWorks/doc/AdventureWorks_2/functions/dbo_ufnGetProductStandardCost_117.html
with the same errors :-(
I think that the idea of ufnGetProductListPrice(ProductID, OrderDate), is to report a SINGLE price or Null if at that given time the price did not exist.
NOT a table or listrow.
Let's just recall the case I supposed before
search OrderDate 1/1/2022
one record with StartDate=1/1/2020 , EndDate=null
one record with StartDate=1/1/2021 , EndDate=null
One idea for such function would be to:
use a subquery to find the MAX() StardDate<#OrderDate for the given #ProductId
use such StartDate to get the correspondig ListPrice in the external query
or give back NULL if we have NO StartDate ListPrice before #OrderDate (e.g #OrderDate=1/1/2019)
What do you think about this query ? Any improvement ?
CREATE FUNCTION dbo.ufnGetProductListPrice (
#ProductID int,
#OrderDate datetime
)
AS
BEGIN
DECLARE #ListPrice money;
SELECT #ListPrice= plph.ListPrice
FROM Production.ProductListPriceHistory plph
WHERE plph.ProductID = #ProductID
AND plhp.StartDate = (
SELECT MAX(StardDate)
FROM Production.ProductListPriceHistory plph1
WHERE plph1.ProductID = #ProductID
AND plhp1.StartDate <= #OrderDate)
RETURN #ListPrice;
END;