--drop constraint se existe
DECLARE
itemExists NUMBER;
BEGIN
     itemExists := 0;

SELECT COUNT(CONSTRAINT_NAME) INTO itemExists 
FROM ALL_CONSTRAINTS
WHERE UPPER(CONSTRAINT_NAME) = UPPER('TARIFA_UNICA');

IF itemExists > 0 THEN
EXECUTE IMMEDIATE 'ALTER TABLE TARIFA DROP CONSTRAINT TARIFA_UNICA';
END IF;

END;