ORA-00036: "maximum number of recursive SQL levels (%s) exceeded"
 *Cause:  An attempt was made to go more than the specified number
          of recursive SQL levels.
 *Action: Remove the recursive SQL, possibly a recursive trigger

CREATE OR REPLACE TRIGGER TRG_CUSTOMER_GROUP_I
BEFORE INSERT 
ON CUSTOMER_GROUP
FOR EACH ROW
DECLARE 
  V_COUNT NUMBER(9);	
BEGIN
  SELECT COUNT(*)
  INTO V_COUNT
  FROM CUSTOMER
  WHERE CUSTOMER_ID =:NEW.CUSTOMER_GROUP_ID;
  IF (V_COUNT = 0) THEN
   	  RAISE_APPLICATION_ERROR(-20100,'error occured
            (CUSTOMER.CUSTOMER_IDDOESNOTEXISTS)'); 
  END IF;
END;
/