Return Minimum Value Of Resultset

SELECT
REF.REFERRAL_TO,
REF.LOCAL_PATIENT_NUM,
REF.REFERRAL_REFERENCE_NUM,
TO_CHAR (REF.REFERRAL_DTE, 'DD/MM/YYYY') REFDATE,
TO_CHAR (OPC.CONTACT_DATE, 'DD/MM/YYYY') CONTACTDATE,
TO_DATE (OPC.CONTACT_DATE, 'DD/MM/YYYY') - TO_DATE (REF.REFERRAL_DTE, 'DD/MM/YYYY')
FROM
PAS.REFERRAL_DETAILS REF
FULL JOIN
PAS.OUTPATIENT_CONTACT OPC

[Code]...

This calculates the difference between a set date and a contact date, I only want it to return the minimum of the results set for each patient, how can this be achieved? At the moment it pulls off several records for each patient as there are several contacts.