case statements and scalar variables

DECLARE
lv_purchase_num NUMBER (3);
lv_rating CHAR (4);
BEGIN
CASE lv_purchase_num
WHEN  lv_purchase_num > 200  THEN lv_rating  := ‘HIGH’;
WHEN lv_purchase_num < 200 AND lv_purchase_num > 100 THEN  lv_rating  := ‘MID’;
ELSE
lv_rating  := ‘LOW’;
END CASE;
DBMS_OUTPUT.PUT_LINE(lv_rating);
END;