A little join problem

Today I found a question in a newsgroup asking for help with creating a join. This join should take a date out of table a and find the record in table b which has a date less than that of table a, but still closest to that date. The tables should be joined through the id column.

Here's my example data (I've changed it a bit from the original request):

mysql> SELECT * FROM a;
+-----+----+------------+
| row | id | d          |
+-----+----+------------+
|   1 | 15 | 2005-12-25 |
|   2 | 20 | 2005-12-29 |
|   3 | 25 | 2006-01-12 |
|   4 | 30 | 2006-01-15 |
+-----+----+------------+
4 rows in set (0.00 sec)

mysql> SELECT * FROM b;
+-----+----+------------+
| row | id | d          |
+-----+----+------------+
|   1 | 15 | 2005-12-19 |
|   2 | 15 | 2005-12-21 |
|   3 | 15 | 2005-12-27 |
|   4 | 20 | 2005-12-11 |
|   5 | 20 | 2005-12-30 |
|   6 | 20 | 2006-01-09 |
|   7 | 25 | 2005-12-31 |
|   8 | 25 | 2006-01-02 |
|   9 | 25 | 2006-01-12 |
|  10 | 25 | 2006-01-16 |
|  11 | 30 | 2006-01-01 |
|  12 | 30 | 2006-01-08 |
|  13 | 30 | 2006-01-13 |
+-----+----+------------+
13 rows in set (0.00 sec)

So for example row 1 from table a should refer to row 2 from table b, because the 21st December is closest, but still less, to 25th December; row 2 from table a should refer to row 4 a.s.o.

For a real MySQL prof this shouldn't be much of a problem, but I like it, because it includes a derived table that's included in a join. This is my solution to this problem:

mysql> SELECT c.row as row_a,
    ->        b.row as row_b,
    ->        c.max_d
    -> FROM b INNER JOIN
    ->   (SELECT a.row, max(b.d) as max_d
    ->    FROM a INNER JOIN b
    ->    ON a.id = b.id AND b.d < a.d
    ->    GROUP BY a.row) AS c
    -> ON b.d = c.max_d;
+-------+-------+------------+
| row_a | row_b | max_d      |
+-------+-------+------------+
|     1 |     2 | 2005-12-21 |
|     2 |     4 | 2005-12-11 |
|     3 |     8 | 2006-01-02 |
|     4 |    13 | 2006-01-13 |
+-------+-------+------------+
4 rows in set (0.00 sec)

If the current date should be included, the only thing that's to be changed would be to alter the < sign to a <= operator. A little but fine query that shows the power of SQL.