SQL表的约束

-- emp表的emptno字段增加外键dept表的deptno


alter table emp add constraint foreign key (deptno) references dept (deptno);


create table t_dept(
id int(8) primary key,
name varchar(200) not null,
emp_max_count int(2) check(emp_max_count >0 and emp_max_count < 51),
parent_id int(8) 
);


alter table t_dept add constraint foreign key (parent_id) references t_dept(id);
-- 字段一定要加括号！