IF Sum

Hi there, 

I'm trying to replicate the following calculation to achieve a conditional sum measure based on a column value from a related table. I need this to me a measure as opposed to a calculated column.

From my understanding, IF statements can only filter based on a measure value as opposed to a column value. 

Ex logic: 

If Category[Category] = X then Sum(Sales[SalesX]) else

If Category[Category] = Y then Sum(Sales[SalesY]) else

If Category[Category] = Z then Sum(Sales[SalesZ]) else 0

The goal is to be able to slice by Category and see the the related Sales Amount. 

Any ideas how this can be accomplished?

Thanks!