create table employees(
employee_id number(6),
last_name varchar2(25) not null,
email varchar2(25),
salary number(8,2),
commission_pct number(2,2),
hire_date date not null,
...
department_id number(4),
constraint emp_dept_fk foreign key (department_id)
references departments(department_id),
constraint emp_email_uk unique(email));
foreign key：在表级指定子表中的列
references：标识在父表中的列
on delete cascade：当父表中的列被删除时，子表中相对应的列也被删除
on delete set null：子表中相应的列置空
，