CASE statements allow you to categorize data that you're interested in -- and exclude data you're not interested in. In order to do this, you can use a CASE statement as a filter in the WHERE statement to remove output you don't want to see.
Here is how you might set that up:
In essence, you can use the CASE statement as a filtering column like any other column in your database. The only difference is that you don't alias the statement in WHERE.