Oracel 修改约束命令

增加修改列的约束：
alter table dep_1
modify
(Dep_id number(5) default 8 constraint dep_1_id_nn not  null) 
                                                                                             －－非空约束                               

修改表的约束
（例如设置主键）
alter table dep_1
add constraint Dep_1_id_pk Primary key(Dep_id)

alter table dep_1
modify
(constraint Dep_1_id_pk Primary key(Dep_id))


（设置外键）
alter table GRADE_1
add constraint grade_dep_1_fk foreign key(DEP) references DEP_1(DEP_ID) ON DELETE CASCADE
                                                                                                                           --当父表记录删除，对应子表记录都删除
alter table GRADE_1
add constraint grade_dep_1_fk foreign key(DEP) references DEP_1(DEP_ID) ON DELETE SET NULL
                                                                                                                          --当父表记录删除，对应子表记录外键置null
（自定义约束check)
alter table dep_1
add constraint Dep_1_nm_len check(length(dep_name) >= 2)

删除表的约束
alter table dep_1
drop
constraint Dep_1_id_pk cascade
                                        －－删除主键，若此主键是其他表的外键，要用cascade参数将那张表的外键一齐删除。