the store proc or command can use "where not exists"
like this
select * from Patients P
where not exists
(select 1 from PatientFiles PF
where attachmentfield = '.pco' and PF.ClientId = P.ClientId)
or you can do this in crystal using a group select process
pull all records into the report (outer joining the two tables)
group on the patients.id field
create a formual field (called 'flag' or whatever you want) to find rows with the '.pco' value
//flag
if table.field = '.pco' then 1 else 0
make sure this formula is set to use "default values for Nulls"
insert a summary as teh SUM of the flag formula field at the patient group footer
SUM(@flag,patient.id)
Now all of your patients missing the pco will have a sum of 0
use that in the group select criteria
SUM(@flag,patient.id)=0