Oracel 修改约束命令

布满荆棘的人生 2022-03-19 12:48 184阅读 0赞

**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。