在查询中写CONSTRAINT名称
请参阅此链接
http://www.techonthenet.com/oracle/foreign_keys/drop.php [ ^ ]

Write the CONSTRAINT name in the query
Refer this link
http://www.techonthenet.com/oracle/foreign_keys/drop.php[^]

更改表t2删除约束[FK_t2_t2]

放置约束[constrint_name]

alter table t2 drop constraint [FK_t2_t2]

drop constraint [constrint_name]