if I understand the issue - you have historic sales data and you want to append current sales data to the file - each file has the same columns (dimensions) -
you would do that with a Union (and you can use a wildcard union ) it can be done in Prep or in tableau proper
see the link
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The issue I am having is in "what is the best way to begin this union with this data".
I downloaded Tableau Prep thinking I could connect to tableau data sources published on the server but that does not seem to be possible with Tableau Prep.
I may have to check with the user but since the prior year is static I might can connect tableau prep to that static TDS file, and then union to a SQL connection that calculates current year.
The main issue is that whenever this refreshes I don't want tableau "wasting it's time" trying to refresh the prior year extract. But if its just pulling everything from a static TDS then possibly that is much faster?
If we connect to a TDS on the local machine, whenever tableau prep attempts to union this data will it refresh if that local machine is off? I presume it would just retain the data.