表已经创建再增加约束的方法

1.not null

alter table table_name modify column_name not null;  

2.default

alter table table_name modify column_name default 'default_value';  

3.primary key

alter table table_name modify column_name primary key; 

or

alter table table_name add constraint constraint_name primary key (column_name);  

4.foreign key

alter table table_name_1 add constraint constraint_name foreign key (column_name_1) references table_name_2 (column_name_2);  

5.check

alter table table_name add constraint constraint_name check (conditions);  

6.unique

alter table table_name add constraint constaint_name unique (column_name);