problem with selects based on dates

Hi everyone

I have a problem with SELECTs using dates and WHERE.
Some examples:

SELECT * FROM emp;
 empno |   ename    |    job     | mgr  |  hiredate  | sal  | comm | deptno
-------+------------+------------+------+------------+------+------+--------
  7499 | ALLEN      | SALESMAN   | 7698 | 1981-02-20 | 1600 |  300 |     30
  7566 | JONES      | MANAGER    | 7839 | 1981-04-02 | 2975 |    0 |     20
  7654 | MARTIN     | SALESMAN   | 7698 | 1981-09-28 | 1250 | 1400 |     30
  7369 | SMITH      | CLERK      | 7902 | 1980-12-17 |  800 |      |
  7934 | MILLER     | CLERK      | 7782 | 1982-01-23 | 1300 |      |     10
  7902 | FORD       | ANALYST    | 7566 | 1981-12-03 | 3000 |      |     20
  7900 | JAMES      | CLERK      | 7698 | 1981-12-03 |  950 |      |     30
  7876 | ADAMS      | CLERK      | 7788 | 1987-05-23 | 1100 |      |     20
  7844 | TURNER     | SALESMAN   | 7698 | 1981-09-08 | 1500 |      |     30
  7788 | SCOTT      | ANALYST    | 7566 | 1987-04-19 | 3000 |      |     20
  7782 | CLARK      | MANAGER    | 7839 | 1981-06-09 | 2450 |      |     10
  7698 | BLAKE      | MANAGER    | 7839 | 1981-05-01 | 2850 |      |     30
  7521 | WARD       | SALESMAN   | 7698 | 1981-02-22 | 1250 |      |     30
  7839 | KING       | PRESIDENT  |      | 1981-11-17 | 5000 |      |     10
(14 rows)

db2001=# SELECT * FROM emp WHERE hiredate > 1981;
 empno |   ename    |    job     | mgr  |  hiredate  | sal  | comm | deptno
-------+------------+------------+------+------------+------+------+--------
  7499 | ALLEN      | SALESMAN   | 7698 | 1981-02-20 | 1600 |  300 |     30
  7566 | JONES      | MANAGER    | 7839 | 1981-04-02 | 2975 |    0 |     20
  7654 | MARTIN     | SALESMAN   | 7698 | 1981-09-28 | 1250 | 1400 |     30
  7934 | MILLER     | CLERK      | 7782 | 1982-01-23 | 1300 |      |     10
  7902 | FORD       | ANALYST    | 7566 | 1981-12-03 | 3000 |      |     20
  7900 | JAMES      | CLERK      | 7698 | 1981-12-03 |  950 |      |     30
  7876 | ADAMS      | CLERK      | 7788 | 1987-05-23 | 1100 |      |     20
  7844 | TURNER     | SALESMAN   | 7698 | 1981-09-08 | 1500 |      |     30
  7788 | SCOTT      | ANALYST    | 7566 | 1987-04-19 | 3000 |      |     20
  7782 | CLARK      | MANAGER    | 7839 | 1981-06-09 | 2450 |      |     10
  7698 | BLAKE      | MANAGER    | 7839 | 1981-05-01 | 2850 |      |     30
  7521 | WARD       | SALESMAN   | 7698 | 1981-02-22 | 1250 |      |     30
  7839 | KING       | PRESIDENT  |      | 1981-11-17 | 5000 |      |     10
(13 rows)

db2001=# SELECT * FROM emp WHERE hiredate > 1981-01-01;
 empno |   ename    |    job     | mgr  |  hiredate  | sal  | comm | deptno
-------+------------+------------+------+------------+------+------+--------
  7499 | ALLEN      | SALESMAN   | 7698 | 1981-02-20 | 1600 |  300 |     30
  7566 | JONES      | MANAGER    | 7839 | 1981-04-02 | 2975 |    0 |     20
  7654 | MARTIN     | SALESMAN   | 7698 | 1981-09-28 | 1250 | 1400 |     30
  7369 | SMITH      | CLERK      | 7902 | 1980-12-17 |  800 |      |
  7934 | MILLER     | CLERK      | 7782 | 1982-01-23 | 1300 |      |     10
  7902 | FORD       | ANALYST    | 7566 | 1981-12-03 | 3000 |      |     20
  7900 | JAMES      | CLERK      | 7698 | 1981-12-03 |  950 |      |     30
  7876 | ADAMS      | CLERK      | 7788 | 1987-05-23 | 1100 |      |     20
  7844 | TURNER     | SALESMAN   | 7698 | 1981-09-08 | 1500 |      |     30
  7788 | SCOTT      | ANALYST    | 7566 | 1987-04-19 | 3000 |      |     20
  7782 | CLARK      | MANAGER    | 7839 | 1981-06-09 | 2450 |      |     10
  7698 | BLAKE      | MANAGER    | 7839 | 1981-05-01 | 2850 |      |     30
  7521 | WARD       | SALESMAN   | 7698 | 1981-02-22 | 1250 |      |     30
  7839 | KING       | PRESIDENT  |      | 1981-11-17 | 5000 |      |     10
(14 rows)

It is not clear to me why in the last query the 1980-12-17 hiredate record
is selected as well.

Mike Withers
University of Western Sydney
Australia