If you are in Excel and just need quick answers done on a periodic basis (eg: not looking for rolling 6 months) you can simply use filters, as follows:
1.) Enable filters on the Transactions sheet column headers
2.) Set the filters on the Date column to any time period you want (in this case the 6 months)
3.) Set the filters on the Categories as desired (either individually or as mentioned pick all 6 categories)
4.) Select the entire Amount column (click the column letter)
5.) Look at the bottom right of the sheet, it will show the SUM value in the footer.
I do not have experience with Google Sheets, but it I imagine it has similar capability.
Advantage of this method is that you can quickly get the Sum amount for any range of date combined with any set of categories, combined with any other columns you want to include as filter criteria.
Thank you kindly.
This works in Google Sheets as well, except for the summing part. I just inserted, at the bottom of the column, a function to sum up the amounts (SUBTOTAL, not SUM, because SUM will add up even the cell contents hidden by the filter whereas SUBTOTAL will not). And it took some fiddling around with the filter criteria to make it work.