
The formula above contains a mixture of row level and aggregate formulae in a single expression which is not allowed in ThoughtSpot. The cost_dollar_amount is an aggregate expression value whereas the customer_key is a row level formula. You can get the desired result by using the formula ‘sum ( cost_dollar_amount)/ count (customer_key) ’.
Incorrect formula
Correct Formula

As for the reasons as to why you should not combine them, once you aggregate the data, you lose the values at the individual row level and hence cannot access them.
Let us take the following example
Class Name Total Marks Class Strength
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Economics 350 8
Math 700 12
Physics 600 9
Let's assume I have a formula as sum(Total Marks) / Class Strength. To calculate the sum, I have aggregated across all the rows and don't have access to the individual rows like 8, 12 and 9 and to be frank that formula doesn't make much sense either. So the formula will have to be corrected to Sum(Total Marks)/ Sum(Class Strength)