How to select changed columns - sql-server

Question

The Problem
I'm trying to detect and react to changes in a table where each update is being recorded as a new row with some values being the same as the original, some changed (the ones I want to detect) and some NULL values (not considered changed).
For example, given the following table MyData, and assuming the OrderNumber is the common value,
ID OrderNumber CustomerName PartNumber Qty Price OrderDate
1 123 Acme Corp. WG301 4 15.02 2020-01-02
2 456 Base Inc. AL337 7 20.15 2020-02-03
3 123 NULL WG301b 5 19.57 2020-01-02
If I execute the query for OrderNumber = 123 I would like the following data returned:
Column OldValue NewValue
ID 1 3
PartNumber WG301 WG301b
Qty 4 5
Price 15.02 19.57
Or possibly a single row result with only the changes filled, like this (however, I would strongly prefer the former format):
ID OrderNumber CustomerName PartNumber Qty Price OrderDate
3 NULL NULL WG301b 5 19.57 NULL
My Solution
I have not had a chance to test this, but I was considering writing the query with the following approach (pseudo-code):
select
NewOrNull(last.ID, prev.ID) as ID,
NewOrNull(last.OrderNumber, prev.OrderNumber) as OrderNumber
NewOrNull(last.CustomerName, prev.CustomerName) as CustomerName,
...
from last row with OrderNumber = 123
join previous row where OrderNumber = 123
Where the function NewOrNull(lastVal, prevVal) returns NULL if the values are equal or lastVal value is NULL, otherwise the lastVal.
Why I'm Looking for an Answer
I'm afraid that the ugly join, the number of calls to the function, and the procedural approach may make this approach not scalable. Before I start down the rabbit hole, I was wondering...
The Question
...are there any other approaches I should try, or any best practices to solving this specific type of problem?

Answer 1

I came up with a solution for the second (less preferred) format:
The Data
Using the following data:
INSERT INTO MyData
([ID], [OrderNumber], [CustomerName], [PartNumber], [Qty], [Price], [OrderDate])
VALUES
(1, 123, 'Acme Corp.', 'WG301', '4', '15.02', '2020-01-02'),
(2, 456, 'Base Inc.', 'AL337', '7', '20.15', '2020-02-03'),
(3, 123, NULL, 'WG301b', '5', '19.57', '2020-01-02'),
(4, 123, 'ACME Corp.', 'WG301b', NULL, NULL, '2020-01-02'),
(6, 456, 'Base Inc.', NULL, '7', '20.15', '2020-02-05');
The Function
This function returns the updated value if it has changed, otherwise NULL:
CREATE FUNCTION dbo.NewOrNull
(
#newValue sql_variant,
#oldValue sql_variant
)
RETURNS sql_variant
AS
BEGIN
DECLARE #ret sql_variant
SELECT #ret = CASE
WHEN #newValue IS NULL THEN NULL
WHEN #oldValue IS NULL THEN #newValue
WHEN #newValue = #oldValue THEN NULL
ELSE #newValue
END
RETURN #ret
END;
The Query
This query returns the history of changes for the given order number:
select dbo.NewOrNull(new.ID, old.ID) as ID,
dbo.NewOrNull(new.OrderNumber, old.OrderNumber) as OrderNumber,
dbo.NewOrNull(new.CustomerName, old.CustomerName) as CustomerName,
dbo.NewOrNull(new.PartNumber, old.PartNumber) as PartNumber,
dbo.NewOrNull(new.Qty, old.Qty) as Qty,
dbo.NewOrNull(new.Price, old.Price) as Price,
dbo.NewOrNull(new.OrderDate, old.OrderDate) as OrderDate
from MyData new
left join MyData old
on old.ID = (
select top 1 ID
from MyData pre
where pre.OrderNumber = new.OrderNumber
and pre.ID < new.ID
order by pre.ID desc
)
where new.OrderNumber = 123
The Result
ID OrderNumber CustomerName PartNumber Qty Price OrderDate
1 123 Acme Corp. WG301 4 15.02 2020-01-02
3 (null) (null) WG301b 5 19.57 (null)
4 (null) ACME Corp. (null) (null) (null) (null)
The Fiddle
Here's the SQL Fiddle that shows the whole thing in action.
http://sqlfiddle.com/#!18/b720f/5/0