From:

http://www.cnblogs.com/xd502djj/archive/2010/11/16/1878392.html

当你要drop一个table时，如果删除table的动作会造成trigger或constraint产生矛盾，系统会出现错误警告的讯息而不会允许执行.。一个极简单的例子,例如你有一个员工基本资料表,上面可能有员工编号和员工姓名等字段,另外有一个员工销售表,上面有员工编号和员工销售额两个字段,员工薪资表的员工编号字段为一个foreign key参照到员工基本资料表的员工编号:

sql> drop table t;

Table dropped.

sql> drop table t1;

sql> create table t (id number,name varchar2(20));

Table created.

sql> create table t1 (id number,sal number);

sql> alter table t add constraint t_pk primary key (id);

Table altered.

sql> alter table t1 add constraint t_fk foreign key (id) references t (id);

sql> insert into t values (1,'JACK');

1 row created.

sql> insert into t values (2,'MARY');

sql> COMMIT;

Commit complete.

sql> insert into t1 values (1,1000);

sql> insert into t1 values (2,1500);

sql> commit;

sql> insert into t1 values (3,200);
insert into t1 values (3,200)
*
ERROR at line 1:
ORA-02291: integrity constraint (SYS.T_FK) violated - parent key not found

(違反了constraint，員工基本資料表根本沒有3號這個員工，何來的銷售紀錄。)

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

(违反了constraint，员工销售表t1有參照到table t，这个reference relation不允许你drop table t)

sql> drop table t cascade constraints;

sql> select * from t1;

ID SAL
---------- ----------
1 1000
2 1500

sql> select CONSTRAINT_NAME,TABLE_NAME from dba_constraints where owner = 'SYS' and TABLE_NAME = 'T1'

no rows selected

sql>

我们可以发现利用Drop table cascade constraints可以以刪除关联table t的constraint來达成你drop table t的目的，原來属于t1的foreign key constraint已经跟随着被删除掉了，但是，储存在table t1的资料可不会被删除，也就是说Drop table cascade constraints 是不影响到存储于objec里的row data。