JOIN condition confusion

Hi,

I am wondering if there is a way to set conditions on a left joined
table without hosing the join altogether:

query = qq |SELECT p.*, a.user_id
FROM patient_dosing p
LEFT JOIN patient_assignment a
ON p.patient_id = a.patient_id
WHERE p.dose_type = 'Missed (AWOL)'
AND (p.dose_date >= $start_date AND p.dose_date <= $end_date)
-- the next two conditions hose the left join
-- AND a.end_date IS NULL
-- AND lower(a.assign_type) = 'primary'
ORDER BY a.user_id, p.patient_id| if ($dbtype ne "oracle");

Thanks much!
Tom