Why Doesn't Coalesce work in an If/Case Statement?

I am trying to write a search that if the field= Email then perform a coalese, but if the field isn't Email- just put in the field- below is what I have written. It seems like coalesce doesn't work in if or case statements. I also tried to accomplishing this with isNull and it also failed. | eval $fieldToMatch$= if($fieldToMatch$==Email, coalesce(Email,NotifyAddress), $fieldToMatch$) Thanks for the help! (essentially i am doing a join on a field and the field could either be email, an ID, etc. but if the field to match on is email- the field NotifyAddress might also contain some emails)