SQL & PL/SQL

Hello Experts,
Is anybody knows how to handle the exception on below MERGE statement?

MERGE INTO COMM_EXSTS_COMIT_AGGR TARGET
  USING
  (
        select  * from   ABC ) SRC
  ON
  (
        SRC.COMMITMENT_ID = TARGET.COMMITMENT_ID 
   )
  
  WHEN MATCHED THEN
  UPDATE 

  WHEN NOT MATCHED THEN
  INSERT ;