Полагаю, проблема в предложении WHERE.
Попробуйте:
a.address = 'Woodstock' OR a.address = 'Roswell';
или:
a.address IN ('Woodstock', 'Roswell');
Пример:
Select a.PatientId,a.name as PatientName,c.treatmentDescription as TreatmentName
FROM patient as a inner join patient-treatment as b
on a.patientID = b.PatientID
INNER JOIN Treatment as c
ON b.TreatmentCode = c.TreatmentCode
WHERE a.address = 'Woodstock' OR a.address = 'Roswell';