GROUP_CONCAT

HI, I NEED THE OUTPUT TO RETURN THE PATIENTS ADMITTE TO THE HOSPITAL MORE THAN ONCE FOR THE YEAR TO DATE. THE QUERY ATTACHED WORKS IN MYSQL BUT WHEN I RUN IT IN ORACLE IT RETURNS THE ERROR MESSAGE THAT GROUP_CONCAT IS INVALID IDENTIFIER. DOES ANYBODY KNOW WHAT IS THE FUNCTION IN ORACLE WHICH ILL RETURN A LIST OF ADMISSIONS FOR EVERY PATIENT?
THANKS

select 
patient.pno, 
patient.psurname, 
group_concat( admission.adno ) as adno_list
from patient, admission, CURRENTDATE
WHERE patient.pno = admission.pno
AND ADMISSION.INDATE  
BETWEEN (currentdate - INTERVAL '1' YEAR)
and currentdate
group by patient.pno, patient.psurname 
having count(*) >= 2;

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