有关外键约束的两个问题。

CREATE TABLE emp
  (empno    NUMBER(4),
   ename    VARCHAR2(10),
   job      VARCHAR2(9),
   mgr      NUMBER(4),
   hiredate DATE,
   sal      NUMBER(7,2),
   comm     NUMBER(7,2),
   deptno   NUMBER(2)   CONSTRAINT fk_deptno
            REFERENCES dept(deptno)
            ON DELETE CASCADE );