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);