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：在表級指定子表中的列
    REFERENCES：標識在父表中的列
    ON DELETE CASCADE：當父表中的列被刪除時，子表中相對應(yīng)的列也被刪除
    ON DELETE SET NULL：子表中相應(yīng)的列置空