Total Credits Without Sort Codes By Primary Provider

SELECT
  p.provcode,
  r.provlname,
  SUM(amount)
FROM
  journal j,
  patients p,
  providers r
WHERE
  j.ptnum=p.ptnum AND
  p.providernum=r.providernum AND
  j.trandate BETWEEN '2000-03-01' AND '2000-03-15' AND
  j.sortcode IS NULL
GROUP BY
  patients.provcode, providers.provlname