I am unsure if this is a "union query", perhaps it is:

I need to join two tables in a way that populates different colums based on the underlying table, and leaving cells in the other table's columns blank where there are no data. I really cannot seem to figure out what type of query is needed for that. Particularly to unite two tables like sales and purchases of the same stock, grouping by client and stock, having one colunm for amount purchased, another for amount sold, a third for purchase price, a fourth for sales price, and displaying each purchase and each sale on separate rows, leaving the irrelevant colunms' cells blank.

Oh, I did this a lot loong time ago, pretty basic, really, but simply cannot seem to tweak my brain to get this together...