1.删除约束  
alter table table_name drop constraint constraint_name;  
2.创建主键  
alter table table_name add constraint constraint_name primary key(column_name1,column_name2);  
3.创建唯一约束  
alter table table_name add constraint constraint_name unique(column_name1,column_name2);  
4.创建外键约束  
alter table table_name1 add constraint constraint_name foreign key(column_name1) references table_name2(column_name1);  
enable/disable：约束/不约束新数据  
novalidate/validate:不对/对老数据进行验证  
alter table table_name add constraint constraint_name check(column_name like 'B%') enable/disable novalidate/validate;  
5.修改约束条件，延时验证，commit时验证  
alter table table_name modify constraint constraint_name initially deferred;  
6.修改约束条件，立即验证  
alter table table_name modify constraint constraint_name initially immediate;  
7.设置约束延迟/立即验证  
alter session set constraints=deferred/immediate;  
8.删除表时删除约束  
drop一个有外键的主键表,带cascade constraints参数级联删除  
drop table table_name cascade constraints;  
9.当truncate外键表时，先将外键设为无效，再truncate;  
truncate table table_name;  
设约束条件无效  
alter table table_name disable constraint constraint_name;  
alter table table_name enable novalidate constraint constraint_name; 

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