SQL> create table a (id number);

Table created.

SQL> alter table a add constraint pk_a primary key(id);

Table altered.

SQL> create table b (bid number references a (id));

Table created.

SQL> insert into a values (1);

1 row created.

SQL> commit;

Commit complete.

SQL> insert into b values (1);

1 row created.

SQL> commit;

Commit complete.

SQL> drop table a ;
drop table a
           *
ERROR at line 1:
ORA-02449: unique/primary keys in table referenced by foreign keys

/*因为有外键对a表进行引用而无法drop。*/

SQL> delete from a;
delete from a
*
ERROR at line 1:
ORA-02292: integrity constraint (SYS.SYS_C005401) violated - child record found

/*因为有外键数据对a表的数据进行引用，数据无法delete*/

SQL> alter table a disable constraint pk_a;
alter table a disable constraint pk_a
*
ERROR at line 1:
ORA-02297: cannot disable constraint (SYS.PK_A) - dependencies exist

/*由于对约束有依赖关系，约束无法disable*/

SQL> alter table b disable constraint SYS_C005401;

Table altered.

SQL> drop table a purge;
drop table a purge
           *
ERROR at line 1:
ORA-02449: unique/primary keys in table referenced by foreign keys

/*即使禁用了外键约束，由于外键约束依然存在，也无法将主表drop。*/

SQL> delete from a ;

1 row deleted.

SQL> commit;

Commit complete.

SQL> delete from b;

1 row deleted.

SQL> commit;

Commit complete.

SQL> drop table a;
drop table a
           *
ERROR at line 1:
ORA-02449: unique/primary keys in table referenced by foreign keys

/*即使主表跟外键表都没有了数据，外键约束被禁用，也无法drop主表。*/

SQL> alter table a disable constraint pk_a;

Table altered.

/*当外键约束被禁用之后，主键约束也能够禁用了。*/

SQL> drop table b;

Table dropped.

SQL> drop table a;

Table dropped.

SQL>
SQL> create table a (id number);

Table created.

SQL> alter table a add constraint pk_a primary key(id);

Table altered.

SQL> create table b (bid number);

Table created.

SQL> alter table b add constraint fk_bid foreign key(bid) references a(id) on delete cascade;

Table altered.

SQL> insert into a values (1);

1 row created.

SQL> commit;

Commit complete.

SQL> insert into b values (1);

1 row created.

SQL> commit;

Commit complete.

SQL> delete from a;

1 row deleted.

SQL> commit;

Commit complete.

SQL> select * from b;

no rows selected

/*级联删除，还有on update cascade 级联更新。*/

结论：
不管我们是禁用约束、删除数据，drop表，都必须先完成外键表的约束禁用、数据删除、表DROP，才能相应的完成主键表的操作。当然如果我们是删除数据的话，也可以先将约束禁用了，然后执行数据删除操作，这个时候可以无序删除。但是如果我们是DROP表的话，必须先DROP外键约束，或者先DROP外键表，才能DROP主键表，光禁用外键约束是不行的。