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

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