Unique counts - across a group by

Question

I am attempting to count unique patients that stay in the hospital. But they want the count by months. I only want the patient to count in the first month. is there a good way to do this.
for example if I have a table like:
CREATE TABLE "PATIENT_CHARGES"
     "SERVICE_DATE" DATE,
     "VISIT_NO" VARCHAR2(20 BYTE)
Insert into PATIENT_CHARGES (SERVICE_DATE,VISIT_NO) values (to_date('01-JAN-12','DD-MON-RR'),'1');
Insert into PATIENT_CHARGES (SERVICE_DATE,VISIT_NO) values (to_date('01-FEB-12','DD-MON-RR'),'5');
Insert into PATIENT_CHARGES (SERVICE_DATE,VISIT_NO) values (to_date('01-FEB-12','DD-MON-RR'),'3');
Insert into PATIENT_CHARGES (SERVICE_DATE,VISIT_NO) values (to_date('01-MAR-12','DD-MON-RR'),'5');
Insert into PATIENT_CHARGES (SERVICE_DATE,VISIT_NO) values (to_date('01-JAN-12','DD-MON-RR'),'2');
Insert into PATIENT_CHARGES (SERVICE_DATE,VISIT_NO) values (to_date('01-FEB-12','DD-MON-RR'),'4');
and I run a query like:
SELECT TO_CHAR(SERVICE_DATE, 'YYYYMM') AS SERVICE_CY_MONTH, COUNT(DISTINCT VISIT_NO)
FROM PATIENT_CHARGES
GROUP BY TO_CHAR(SERVICE_DATE, 'YYYYMM')
I get:
201201     2
201202     3
201203     1
My total is 6. But I only have 5 patients. The 5th person stayed Feb - March . So although its a distinct count, it is by month, so they get double counted. What's my best way to get around this?

Answer 1

Frank Kulash wrote:
Hi,
br**** wrote:
Sorry its not really a unique patient identifier - that would be a medical record number, this is more a counter for each visit they have.Okay, so where I used visit_no, you should use whatever identifies the patient.Frank:
Hopitals are odd places. In many ways, we do not care about patients, only visits (well, at least for counting/billing purposes). The medical reord number (MRN) uniquely identifies a patient across all of their interactions with the hospital, while a visit number uniquely identifies a particular interaction with the hospital. In most of the hopital systems I have seen patient is an attribute of visit, not the other way around.
Certainly in my hospital, we would count distinct visits rather than patients. That is, if a patient with MRN 123 was here on visit number 9865 in February, and again on visit number 10023 in March, we would count that twice, but if visit 10023 extended into April, it would depend on the specific question being asked whehter we aould count that as one or two.
John