Case:

User creates a patient set. User then uses that patient set as a filter on a subsequent query. The following SQL is generated and produces an error.

=====
select f.patient_num
from i2b2crc.observation_fact f
where
f. patient_num IN (select patient_num from i2b2crc.qt_patient_set_collection where result_instance_id = )
group by f.patient_num
=====
This fails because the where clause is incomplete.