Access 2013 SQL to perform linear interpolation where necessary

Question

I have a database in which there are 13 different products, sold in 6 different countries.

Prices increase once a year.

Prices need to be calculated using a linear interpolation method.  I have 21 different price and quantity increments for each product for each country for each year.

The user needs to be able to see how much an order would cost for any given value (as you would expect).

What the database needs to do (in English!) is to:

If there is a matching quantity from TblOrderDetail in the TblPrices, use the price for the current product, country and year

if there isn't a matching quantity but the quantity required is greater than 1000 for one product (GT) and greater than 100 for every other product: Find the highest quantity for the product, country and year (so, 1000 or 100, depending on the product), and calculate a pro-rated price.  eg.  If someone wanted 1500 of product GT for the UK for 2015, we'd look at the price for 1000 GT in the UK for 2015 and multiply it by 1.5.  If 1800 were required, we'd multiply it by 1.8.  I haven't been able to get this working yet as I'm looking at it alongside the formula for the next possibility...

If there isn't a matching quantity and the quantity required is less than 1000 for the product GT but 100 for the other products (this is the norm)... Find the quantity and price for the increment directly below the quantity required by the user for the required product, country and year (let's call these quantitybelow and pricebelow)

Find the quantity and price for the increment directly above the quantity required by the user for the required product, country and year (let's call these quantityabove and priceabove)

Calculate the price for the required number of products for an account holder in a particular country for a given year using this formula.

ActualPrice: PriceBelow + ((PriceAbove - PriceBelow) * (The quantity required in the order detail - QuantityBelow) / (QuantityAbove - QuantityBelow))

I have spent days on this and have sought advice about this before but I am still getting very stuck.

The tables I've been working with to try and make this work are as follows:

TblAccount (primary key is AccountID, it also has a Country field which joins to the TblCountry.Code (primary key)

TblOrders (primary key is Order ID) which joins to TblAccount via the AccountID field; TblOrderDetail via the OrderID.  This table also holds the OrderDate and Recipient ID which links to a person in TblContact - I don't need that here but will need it later to generate an invoice 

TblOrderDetail (primary key is DetailID) which joins to TblOrders via OrderID field; TblProducts via ProductID field, and holds the Quantity required as well as the product

TblProducts (primary key is ProductCode) which as well as joining to TblOrderDetail, also joins to TblPrice via the Product field

TblPrices links to the TblProducts (as you have just read).  I've also created an Alias for the TblCountry (CountryAliasForProductCode) so I can link it to the TblPrices to show the country link. I'm not sure if I needed to do this - it doesn't work if I do or I don't do it, so I seek guidance again here.

This is the code I've been trying to use (and failing) to get my price and quantity steps above and I hope to replicate it, making a couple of tweaks to get the steps below:

SELECT MIN(TblPrices.stepquantity) AS QuantityAbove, MIN(TblPrices.StepPrice) AS PriceAbove, TblOrders.OrderID, TblOrders.OldOrderID, TblOrders.AccountID, TblOrders.OrderDate, TblOrders.RecipientID, TblOrders.OrderStatus, TblOrderDetail.DetailID, TblOrderDetail.Product, TblOrderDetail.Quantity

FROM (TblCountry INNER JOIN ((TblAccount INNER JOIN TblOrders ON TblAccount.AccountID = TblOrders.AccountID) INNER JOIN (TblOrderDetail INNER JOIN TblProducts ON TblOrderDetail.Product = TblProducts.ProductCode) ON TblOrders.OrderID = TblOrderDetail.OrderID) ON TblCountry.Code = TblAccount.Country) INNER JOIN (TblCountry AS CountryAliasForProduct INNER JOIN TblPrices ON CountryAliasForProduct.Code = TblPrices.CountryCode) ON TblProducts.ProductCode = TblPrices.Product

WHERE (StepQuantity >= TblOrderDetails.Quantity)
AND (TblPrices.CountryCode = TblAccount.Country)
AND (TblOrderDetail.Product = TblPrices.Product)
AND (DATEPART('yyyy', TblPrices.DateEffective) = DATEPART('yyyy', TblOrders.OrderDate));

I've also tried... I've even tried going back to basics and trying again to generate the steps below in 1 query, then try the steps above in another and finally, create the final calculation in another query.

This is what I have been trying to get my prices and quantities below:

SELECT Max(StepQuantity) AS quantity_below, Max(StepPrice) AS price_below, TblOrderDetails.Quantity, TblAccounts.Country
FROM 
(TblProducts INNER JOIN TblPrices ON TblProducts.ProductCode = TblPrices.Product)
(TblOrderDetail INNER JOIN TblProducts ON TblOrderDetail.Product = TblProducts.ProductCode)
(TblOrders INNER JOIN TblOrderDetail ON TblOrders.OrderID = TblOrderDetail.OrderID)
(TblAccount INNER JOIN TblOrders ON TblAccount.AccountID = TblOrders.AccountID),
WHERE (((TblPrices.StepQuantity)<=(TblOrderDetail.Quantity)) AND ((TblPrices.CountryCode)=([TblAccounts].[country])) AND ((TblPrices.Product)=([TblOrderDetail].[product])) AND ((DatePart('yyyy',[TblPrices].[DateApplicable]))=(DatePart('yyyy',[TblOrders].[OrderDate]))));

You may be able to see glaring errors in this but I'm afraid I can't.  I've tried re-jigging it and I'm getting nowhere.

I need to be able to tie the information in to the OrderDetail records as the price generated will need to be added to a financial transactions table as a debit amount and will show as an amount owing on statements.

I'm really not very good at SQL.  I've read and worked though several self-study books and I have asked part of this question before; but I really am struggling with it.  If anyone has any ideas on how to proceed, or even where I've gone wrong with my code, I'd be delighted, even if you tell me I shouldn't be using SQL. For the record, I originally posted this question on a different forum under Visual Basic. Responses from that forum brought me to SQL - however, anything that works would be good!

I've even tried, using Excel, concatenating the Year&Product&Country&Quantity to get a unique product code, interpolating the prices for every quantity between 1 and 1000 for each product, country and year and bringing them into a TblProductsAndPrices table. In Access, I created a query to concatenate the Year(of order date from tblOrders)&Product(of tblorderdetails)&Country(of tblAccount) in order to get the required product code for the order. Another query would find a price for me. However, any product code that doesn't appear on the list (such as where a quantity isn't listed in the tblProductsAndPrices as it is larger than the highest price increment) doesn't have a price.

If there was a workable solution to what I've just described that would generate a price for everything, then I'd be so pleased.

I'd really like to be able to generate an order for any quantity of any product for any account based in any country on any date and retrieve a price which will be used to "debit" a financial account in the database, who in a transaction history for an account and appear on statements. I'd also like to be able to do an ad-hoc price check on the spot.

Thank you very much for taking the time to read this.  I really appreciate it. If you could offer any help or words of encouragement, I'd be very grateful.

Many thanks Karen

Answer 1

Maybe no one thinks on an easy solution to the problem, since not all minds work in database thinking.

Easy solution: Create one view that gives all calculated values, not only the final one you need, each one as a column. Then you can use such view in a relation view and use on some rows one of the values and on other rows other values, etc.

How to think is simple, think in reverse order, instead of thinking "if that then I need to calculate such else I need this other", think as "I need "such" and I need "this other", both are columns of an intermediate view, then think on top level "if" that would be another view, such view will select the correct value ignoring the rest.

Never ever try to solve all in one step, that can be a really big headache.

Pros: You can isolate calculated values (needed or not), sql is much more easy to write and maintain.

Cons: Resources use is bigger than minimal, but most of times that extra calculated values does not represent a really big impact.

In terms of tutorial out there: Instead of a Top-Down method, use a Down-Top method.

Sometimes it is better (with your example) to calculate all three values (you write sentences on bold) ignoring the if part, and have all three possible values for your order and after that discard the ones not wanted, than trying to only calculate one.

Trying to calculate only one is thinking as a procedural programming, when working with databases most times one must get rid of such thinking and think as reverse, first do the most internal part of such procedural programming to have all data collected, then do the external selection of the procedural programing.

Note: If one of the values can not be calculated, just generate a Null.

I know it is hard to think on First in, last out (Down-Top) model, but it is great for things as the one you want.

Step1 (on specific view, or a join from one view per calculation):

• Calculate column 1 as price for the current product, country and year
• Calculate column 2 as calculate a pro-rated price as if 1000
• Calculate column 3 as calculate a pro-rated price as if 100
• Calculate column 4 as etc
• Calculate column N as etc

Step 2 (Another view, the one you want):

• Calculate the if part, so you can choose adequate column from previous view (you can use immediately if or a calculated auxiliary field).

Hope you can follow theese way of thinking, I have solved a lot of things like that one (and more complex) thinking in that way, but it is not easy to think as that, needs an extra effort.