Count of Patients and Services by Provider, Pt Category, Service Code

Give me a count of unique patients and a count of services rendered, grouped by provider, patient category, and service code for a specified date range.

SELECT
  (a.provfname + ' ' + a.provlname) AS "Provider",
  c.categcode AS "Category",
  a.srvcode AS "Service",
  COUNT(DISTINCT a.ptnum) AS "Pt Count",
  COUNT(DISTINCT a.jnum) AS "Srv Count"
FROM
  sos.rv_charges a
  JOIN sos.patients b ON a.ptnum = b.ptnum
  LEFT OUTER JOIN sos.ptcategs c ON b.ptcategnum = c.ptcategnum
WHERE
  a.trandate BETWEEN '2008-01-01' AND '2008-12-31'
GROUP BY
  "Provider","Category","Service"