Active Patients Without a Diagnosis

I am interested in running a report based on the active patients who  currently lack a Diagnosis code on their records. I currently have a flag prior to billing claims, but I want an actual print up of the patients who lack a dx.

SELECT
  a.lastname, a.firstname, a.id
FROM
  sos.patients a
  LEFT OUTER JOIN sos.ptcsudx b ON a.ptcsunum = b.ptcsunum
WHERE
  a.flag = 0
  AND a.dischargedate IS NULL
  AND b.dxcode1 IS NULL
ORDER BY a.lastname, a.firstname, a.id