A PL/SQL function that formats an Oracle INTERVAL YEAR TO MONTH value to a conformant ISO 8601 string.

format_iso_yminterval.plsql

CREATE OR REPLACE FUNCTION format_iso_yminterval(interval_in INTERVAL YEAR TO MONTH) RETURN VARCHAR2
IS 
  iso_out VARCHAR2(14) := 'P';
  year NUMBER;
  month NUMBER;
  
BEGIN
    year := EXTRACT(YEAR FROM interval_in);
  IF year <> 0 THEN 
     iso_out := iso_out || year || 'Y';
  END IF;
  month := EXTRACT(MONTH FROM interval_in);
  IF month <> 0 THEN 
     iso_out := iso_out || month || 'M';
  END IF;
  IF iso_out = 'P' THEN 
    iso_out := iso_out || '0Y0M';
    END IF;
  
  RETURN iso_out;  
END;