I am looking at an old stored procedure that's job is to preserve the New sort order based on yesterday's and today's data.

Sort orders are not being preserved any longer and I have narrowed it down to the WHERE clause eliminating all rows. The main goal is to preserve the SortOrder so if some custom data was in position 4 yesterday, any NEW custom data that takes its place should ALSO have position 4.

If I eliminate

--AND b.PrimaryID = b.SortOrder
then I get thousands of rows. I suspect something is wrong but it I am not understanding. How can I make this simpler so it is REALLY easy to understand?

IMPORTANT: the SortOrder actually equals the PrimaryID if the data is no longer sorted. Otherwise it is incremental 1 2 3 4 5 6 7 .. and so on. I guess this was the original architects way of doing it.

-- Merge data and get missing rows that have not changed.
SELECT
PrevPrimaryID = a.PrimaryID
,a.WidgetID
,a.AnotherValue
,a.DataID
,PrevSortOrder = a.SortOrder
,NewPrimaryID = b.PrimaryID
,NewDataID = b.DataID
,NewStartDate = b.StartDate
,NewSortOrder = b.SortOrder
INTO #NewOrder2
FROM #YesterdaysData2 a
LEFT JOIN #TodaysData2 b ON a.WidgetID = b.WidgetID
AND a.AnotherValue = b.AnotherValue
WHERE
a.Primaryid <> a.sortorder
AND b.PrimaryID = b.SortOrder

SELECT * FROM #NewOrder2

-- later update based on #NewOrder2...