I have a planning structure on two tables to store available slots by day, and sessions.

A slot is defined by a range of time in the day.

CREATE TABLE slot (
  `id` int(11) NOT NULL AUTO_INCREMENT
, `date` date
, `start` time
, `end` time
);

Sessions can't overlap themselves and must be wrapped in a slot.

CREATE TABLE session (
  `id` int(11) NOT NULL AUTO_INCREMENT
, `date` date
, `start` time
, `end` time
);

I need to generate a list of available blocks of time of a certain duration, in order to create sessions.

Example:

INSERT INTO slot
  (date, start, end)
VALUES
  ("2010-01-01", "10:00", "19:00")
, ("2010-01-02", "10:00", "15:00")
, ("2010-01-02", "16:00", "20:30")
;

INSERT INTO slot
  (date, start, end)
VALUES
  ("2010-01-01", "10:00", "19:00")
, ("2010-01-02", "10:00", "15:00")
, ("2010-01-02", "16:00", "20:30")
;

2010-01-01

   <##><####>                               <- Sessions
 ------------------------------------       <- Slots
10  11  12  13  14  15  16  17  18  19  20

2010-01-02

         <##########>          <########>   <- Sessions
 --------------------    ------------------ <- Slots
10  11  12  13  14  15  16  17  18  19  20

I need to know which spaces of 1 hour I can use:

+------------+-------+-------+
| date       | start | end   |
+------------+-------+-------+
| 2010-01-01 | 13:00 | 14:00 |
| 2010-01-01 | 14:00 | 15:00 |
| 2010-01-01 | 15:00 | 16:00 |
| 2010-01-01 | 16:00 | 17:00 |
| 2010-01-01 | 17:00 | 18:00 |
| 2010-01-01 | 18:00 | 19:00 |
| 2010-01-02 | 10:00 | 11:00 |
| 2010-01-02 | 11:00 | 12:00 |
| 2010-01-02 | 16:00 | 17:00 |
+------------+-------+-------+