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