mysql> select * from corfu44_reservations;
+----+--------+-----------+-------+---------------------+---------------------+-------------+-----------+----------+--------+--------+----------+-----
--------+-------------+
| id | car_id | client_id | agent | start_datetime      | end_datetime        | start_place | end_place | contract | price1 | price2 | babyseat | boos
terseat | remarks     |
+----+--------+-----------+-------+---------------------+---------------------+-------------+-----------+----------+--------+--------+----------+-----
--------+-------------+
|  7 |      1 |         1 | agent | 2011-04-11 00:00:00 | 2011-04-16 00:00:00 |             |           | test     | 10     | 20     | yes      | yes
        | test remark |
+----+--------+-----------+-------+---------------------+---------------------+-------------+-----------+----------+--------+--------+----------+-----
--------+-------------+
1 row in set (0.00 sec)

mysql> SELECT id FROM corfu44_reservations WHERE (str_to_date('2011-04-11 10:00:00','%Y-%c-%d %H:%i:%s') BETWEEN start_datetime AND end_datetime) OR (
str_to_date('2011-04-12 12:00:00','%Y-%c-%d %H:%i:%s') BETWEEN start_datetime AND end_datetime) AND car_id = 1
    -> ;
+----+
| id |
+----+
|  7 |
+----+
1 row in set (0.00 sec)