how to transfer this iterative solution to a set-based solution - sql

Question

/*
Stored Procedure
April 30, 2021
Mohamad Chaker
*/
USE CIS111_BookStoreMC
GO
--drop the procedure
IF OBJECT_ID('spAssetInfo') IS NOT NULL
DROP PROC spAssetInfo
GO
--Create the stored procedure
CREATE PROC spAssetInfo
AS
--Create a temporary table to display the inventory
SELECT AssetID, Description, Cost, PurchaseDate
INTO #temptable
FROM Assets
--Add a new column to the temporary table displaying the date an asset is completely depreciated
ALTER TABLE #temptable ADD CurrentValue MONEY
DECLARE #years INT;
DECLARE #currentYear INT;
SET #currentYear = YEAR(getdate());
DECLARE #cost MONEY;
--Add a new column to the temporary table to display the current value of the item in the current year
ALTER TABLE #temptable ADD CompleteDepreciationYear DATE
--#value holds the cost of an asset and is used to check when the value of an asset drops low
DECLARE #value MONEY;
SET #value = 0.00;
--#counter is an int that I used to iterate over the table rows
DECLARE #counter INT;
--#depreciationNum holds the amount of years until an item is completely depreciated to be later used in DATEADD()
DECLARE #depreciationNum INT;
SET #counter = 1;
DECLARE #assetsTableSize INT;
SET #assetsTableSize = (SELECT COUNT(*) FROM Assets);
WHILE (#counter <= #assetsTableSize)
BEGIN
--Current Value
SET #years = #currentYear - (select YEAR(PurchaseDate) From Assets Where AssetID = #counter);
SET #cost = (select Cost From Assets Where AssetID = #counter);
--calculate current value of each asset
WHILE(#years>0)
BEGIN
SET #cost = #cost * 0.8;
SET #years = #years - 1;
END
--add the current value of each asset to the temporary table
UPDATE #temptable
SET CurrentValue = #cost
WHERE AssetID = #counter;
--Deprection Year
SET #depreciationNum = 0;
SET #value = (select Cost From Assets Where AssetID = #counter);
WHILE(#value >0.1)
BEGIN
SET #value = #value * 0.8;
SET #depreciationNum = #depreciationNum + 1;
END
--add the date each asset is completely depreciated to the temporary table
UPDATE #temptable
SET CompleteDepreciationYear = CAST(DATEADD(year, #depreciationNum, (select PurchaseDate From Assets Where AssetID = #counter)) AS DATE)
WHERE AssetID = #counter;
--increment the counter
SET #counter = #counter + 1;
END
--display the assets inventory
SELECT * FROM #temptable
Prompt: Show an asset inventory along with the current value (minus 20% depreciation per year). Also show the year when each item will be
completely depreciated.
basically I am trying to show an Asset Inventory with PurchaseDate and the date the item is completely depreciated, the asset depreciates 20% per year. I tried to do a temporary table and copy some of the assets table columns to it then adding a column for the date when the asset completely depreciates.
I implemented this using an iterative solution but I was advised to post on SO to try and do this using a set-based implementation. I am new to SQL and newly learned that it's a set-based language and that it isn't very good with iterative solutions.
Thank you in advance!

Answer 1

The formula to calculate the current value is:
CurrentValue = InitialValue * (1-0.2)^(Today - InitialDate)
To find the NumberOfYears that the asset will reach 0.1:
0.1 = CurrentValue * (1-0.2)^NumberOfYears. Taking the log from both sides:
Log(0.1) = Log(CurrentValue * Log(0.8^NumberOfYears)
Log(0.1) = Log(CurrentValue) + Log(0.8^NumberOfYears)
Log(0.1) = Log(CurrentValue) + NumberOfYears * Log(0.8)
[Log(0.1) - Log(CurrentValue)] / Log(0.8) = NumberOfYears
You can create a function that use this formula and returns the NumberOfYears:
CREATE FUNCTION GetNumberOfYears(
#InitialValue FLOAT
)
RETURNS FLOAT
AS
BEGIN
DECLARE #CurrentValue FLOAT,
#InitialValue FLOAT,
#YearsToZero FLOAT;
-- CurrentValue Formula
SET #CurrentValue = #InitialValue * POWER(CAST(0.8 AS FLOAT), YEAR(GETDATE() - YEAR(PurchaseDate));
-- NumberOfYears Formula
SELECT #YearsToZero = (LOG(0.1) - LOG(#CurrentValue)) / LOG(0.8);
RETURN #YearsToZero;
END
It will give you the number of years (e.g. 54.6724159 years).
Then use it like SELECT GetNumberOfYears(Cost) FROM Assets
Or, you can use directly from your SELECT:
SELECT
Cost * POWER(CAST(0.85 AS FLOAT), YEAR(GETDATE()) - YEAR(PurchaseDate)) AS CurrentValue,
(LOG(0.1) - LOG( [the above CurrentValue formula] )) / LOG(0.8) AS YearsToZero
FROM Assets