How to Exclude NULL Values in a CASE WHEN Statement in PQL?

Hi All,

When using a case when statement how would I go about excluding all the values which are not true from the result.

Lets say:
CASE WHEN PROCESS EQUELS 'A' -> 'B'
THEN 'A to B'
WHEN PROCESS EQUELS 'C' -> 'D'
THEN 'C to D'

Now I'm only interested in plotting the values fall in the 'buckets' basically. How do I exclude the rest which are NULL (very large number of cases).