SQL> create table a (aid integer, constraint a_pk primary key (aid));

Table created.

SQL> create table b (bid integer, aid integer,
  2  constraint b_pk primary key (bid),
  3  constraint b_fk foreign key(aid) references a);

Table created.

SQL> select constraint_name from user_constraints where table_name='B';

CONSTRAINT_NAME
------------------------------
B_FK
B_PK

SQL> insert into a values (1);

1 row created.

SQL> insert into b values (11,1);

1 row created.

SQL> alter table a drop constraint a_pk;
alter table a drop constraint a_pk
                              *
ERROR at line 1:
ORA-02273: this unique/primary key is referenced by some foreign keys


SQL> alter table a drop constraint a_pk CASCADE;

Table altered.

SQL> select constraint_name from user_constraints where table_name='B';

CONSTRAINT_NAME
------------------------------
B_PK