Finding 'free' times in MySQL
- by James Inman
Hi,
I've got a table as follows:
mysql> DESCRIBE student_lectures;
+------------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------+----------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| course_module_id | int(11) | YES | MUL | NULL | |
| day | int(11) | YES | | NULL | |
| start | datetime | YES | | NULL | |
| end | datetime | YES | | NULL | |
| cancelled_at | datetime | YES | | NULL | |
| lecture_type_id | int(11) | YES | | NULL | |
| lecture_id | int(11) | YES | | NULL | |
| student_id | int(11) | YES | | NULL | |
| created_at | datetime | YES | | NULL | |
| updated_at | datetime | YES | | NULL | |
+------------------+----------+------+-----+---------+----------------+
I'm essentially wanting to find times when a lecture doesn't happen - so to do this I'm thinking a query to group overlapping lectures together (so, for example, 9am-10am and 10am-11am lectures will be shown as a single 9am-11am lecture). There may be more than two lectures back-to-back.
I've currently got this:
SELECT l.start, l2.end
FROM student_lectures l
LEFT JOIN student_lectures l2 ON ( l2.start = l.end )
WHERE l.student_id = 1 AND l.start >= '2010-04-26 09:00:00' AND l.end <= '2010-04-30 19:00:00' AND l2.end IS NOT NULL AND l2.end != l.start
GROUP BY l.start, l2.end
ORDER BY l.start, l2.start
Which returns:
+---------------------+---------------------+
| start | end |
+---------------------+---------------------+
| 2010-04-26 09:00:00 | 2010-04-26 11:00:00 |
| 2010-04-26 10:00:00 | 2010-04-26 12:00:00 |
| 2010-04-26 10:00:00 | 2010-04-26 13:00:00 |
| 2010-04-26 13:15:00 | 2010-04-26 16:15:00 |
| 2010-04-26 14:15:00 | 2010-04-26 16:15:00 |
| 2010-04-26 15:15:00 | 2010-04-26 17:15:00 |
| 2010-04-26 16:15:00 | 2010-04-26 18:15:00 |
...etc...
The output I'm looking for from this would be:
+---------------------+---------------------+
| start | end |
+---------------------+---------------------+
| 2010-04-26 09:00:00 | 2010-04-26 13:00:00 |
| 2010-04-26 13:15:00 | 2010-04-26 18:15:00 |
Any help appreciated, thanks!
