我不明白为什么我的查询没有返回任何内容：
select * from emp where hiredate between to_date ('01/01/81',
'mm/dd/yy') and to_date ('12/31/81', 'mm/dd/yy');
该表的结构如下：
CREATE TABLE EMP (
EMPNO NUMBER(4) NOT NULL,
ENAME CHAR(10),
JOB CHAR(9),
MGR NUMBER(4) CONSTRAINT EMP_SELF_KEY
REFERENCES EMP (EMPNO),
HIREDATE DATE,
SAL NUMBER(7,2),
COMM NUMBER(7,2),
DEPTNO NUMBER(2) NOT NULL,
CONSTRAINT EMP_FOREIGN_KEY FOREIGN KEY (DEPTNO) REFERENCES DEPT (DEPTNO),
CONSTRAINT EMP_PRIM_KEY PRIMARY KEY (EMPNO));
INSERT INTO EMP VALUES (7839,'KING','PRESIDENT',NULL,'17-NOV-1981',5000,NULL,10);
INSERT INTO EMP VALUES (7698,'BLAKE','MANAGER',7839,'1-MAY-1981',2850,NULL,30);
INSERT INTO EMP VALUES (7782,'CLARK','MANAGER',7839,'9-JUN-1981',2450,NULL,10);
INSERT INTO EMP VALUES (7566,'JONES','MANAGER',7839,'2-APR-1981',2975,NULL,20);
INSERT INTO …