MySQL中模拟oracle中的rownum列

mysql> select @rn := @rn + 1 as rownum, emp_no, dept_no, from_date, to_date
    ->   from dept_emp, (select @rn := 0) a limit 20;
+--------+--------+---------+------------+------------+
| rownum | emp_no | dept_no | from_date  | to_date    |
+--------+--------+---------+------------+------------+
|      1 |  10001 | d005    | 1986-06-26 | 9999-01-01 |
|      2 |  10002 | d007    | 1996-08-03 | 9999-01-01 |
|      3 |  10003 | d004    | 1995-12-03 | 9999-01-01 |
|      4 |  10004 | d004    | 1986-12-01 | 9999-01-01 |
|      5 |  10005 | d003    | 1989-09-12 | 9999-01-01 |
|      6 |  10006 | d005    | 1990-08-05 | 9999-01-01 |
|      7 |  10007 | d008    | 1989-02-10 | 9999-01-01 |
|      8 |  10008 | d005    | 1998-03-11 | 2000-07-31 |
|      9 |  10009 | d006    | 1985-02-18 | 9999-01-01 |
|     10 |  10010 | d004    | 1996-11-24 | 2000-06-26 |
|     11 |  10010 | d006    | 2000-06-26 | 9999-01-01 |
|     12 |  10011 | d009    | 1990-01-22 | 1996-11-09 |
|     13 |  10012 | d005    | 1992-12-18 | 9999-01-01 |
|     14 |  10013 | d003    | 1985-10-20 | 9999-01-01 |
|     15 |  10014 | d005    | 1993-12-29 | 9999-01-01 |
|     16 |  10015 | d008    | 1992-09-19 | 1993-08-22 |
|     17 |  10016 | d007    | 1998-02-11 | 9999-01-01 |
|     18 |  10017 | d001    | 1993-08-03 | 9999-01-01 |
|     19 |  10018 | d004    | 1992-07-29 | 9999-01-01 |
|     20 |  10018 | d005    | 1987-04-03 | 1992-07-29 |
+--------+--------+---------+------------+------------+
20 rows in set (0.00 sec)

mysql>