need help joining two tables with a having count clause
very close only the GROUP BY Columns forgotten

Try this:

select 
PTC_DIAGNOSIS.PATIENT_ID, 
PT_BASIC.PATIENT_CODE, 
PT_BASIC.NAME_FIRST, 
PT_BASIC.NAME_LAST 
from PTC_DIAGNOSIS 
inner join PT_BASIC on PTC_DIAGNOSIS.PATIENT_ID=PT_BASIC.PATIENT_ID
where PTC_DIAGNOSIS.create_date&gt;'12/01/2015' 
group by 
PTC_DIAGNOSIS.PATIENT_ID, 
PT_BASIC.PATIENT_CODE, 
PT_BASIC.NAME_FIRST, 
PT_BASIC.NAME_LAST 
having COUNT(*)=1