Left Outer Join Not Working?

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You should move the constraints on prescriptions.filldate into the ON condition of the join, and remove it from the where clause:

LEFT OUTER JOIN prescriptions ON prescriber.dea_no = prescriptions.dea_no
                             AND prescriptions.filldate >= '09-01-12'
                             AND prescriptions.filldate <= '09-17-12'

Otherwise, entries for which there are no prescriptions end up with nulls in prescriptions.filldate, and the WHERE clause throws them away.

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