I have imported my transactional data as a flat table using a numbered list. Something like this:

I want the user to go into a second module where they can select drop-downs for City, and it will sum the sales. This second mapping module must be a flat table too (numbered list) because if I were to dimensionalize it, the model would explode (my real data has many more attributes than just City).

How can I SUM the Sales into the second module? I get an error when using SUM, because there are no list dimensions in the 2nd table except for the numbered list of Mapping.