Oracle: DROP TABLE … CASCADE CONSTRAINT

create table tq84_A (
   id number primary key
);

create table tq84_B (
   id   number primary key,
   id_a        references TQ84_A
);

alter table tq84_A add id_b references tq84_B;

--
--   The following statements both cause ORA-02449: unique/primary keys in table referenced by foreign keys:
--
drop table tq84_A;
drop table tq84_B;

--
--   Tables can be dropped with DROP TABLE … CASCADE CONSTRAINTS:
--
drop table tq84_A cascade constraints;
drop table tq84_B cascade constraints;