E-Mail Addresses For Active Patients

I need a list of active patients with their email addresses. Only list those with emails, and sort in order by name.

SELECT DISTINCT
a.lastname, a.firstname, b.email
FROM
sos.patients a JOIN payors b ON a.payornum = b.payornum
WHERE
flag = 0 AND dischargedate IS NULL AND email > ''
ORDER BY
a.lastname, a.firstname, b.email