Error in dataflow transform


I am looking to store a couple values in variables, to avoid clutter, of my MYSQL transform and am running into problems.  How can I set variables in a transform and then use later in that transform? 


Here is what I have currently and then I call these variables later in the transform...

-- Variables of actions taking place.
SET @finalcheck := max(case when `action` = 'Checked Final' then `date` end)
SET @exin := max(Case when `action` = 'Exhibits In' then `date` end)
SET @exscan := max(case when `action` = 'Scanned Exhibits' then `date` end)
SET @exdone := max(Case when `action` = 'Exhibits Done' then `date` end)
,timestampdiff(hour, @exin,@exdone) as `ExProcessingHours`