Assigning specific data to a specific column value in pivot table

I'm trying to figure out how to create a category/subcategory feature in my pivot table. The idea is have all the 15 or so main categories displayed on the left hand column in whichever order, and then having each group of subcategories right below their corresponding main categories. I have a conditional dropdown list for my monthly transactions table, where if I select category Travel, the next tab over will only allow me to select subcategories such as Gas, Plane, or Train, which is nice. However, I don't want to use my transactions table as the data source for my pivot table because not all categories and sub categories are in use. If that makes sense. I want to be able to see all subcategories for a particular category, whether or not there was an expense made that month that falls into that subcategory. I also have on a different worksheet a main category table, and individual sub category tables which are all connected to my monthly transactions table. This solves my problem of not having all categories/subcategories display, but its still not a pivot table. The problem with using all of these multiple tables to create a pivot table is obvious: once I drag the Travel subcategory into the column box underneath Category, the Travel subcategory will display itself under ALL categories. Dragging more subcategories underneath them makes the pivot table look even more convoluted. Any help or light shed on my particular problem and how to solve it(if there is a way) would be helpful. I can always provide more information about my excel spreadsheet if needed.