mysql select rows by consecutive date - mysql

Question

I have a table of available date blocks (7 days in my case) which may or may not be consecutive:
start_date end_date booked id room_id
2012-07-14 2012-07-21 0 1 6
2012-07-21 2012-07-28 0 2 6
2012-07-28 2012-08-04 1 3 6
2012-08-04 2012-08-11 0 4 6
What I'd like to do is be able to get a result set that gives me one row per X weeks of consecutive unbooked dates, within a date range.
So, for 2 week blocks starting on the 14th of July and using the above table data, I would expect the following:
start_date end_date booked
2012-07-14 2012-07-28 0
The second block of 2 weeks would not be returned as one of the component weeks is booked.
Here are a few ideas I've tried already:
SELECT
MIN(start_date) AS start_date_min,
MAX(end_date) AS end_date_max,
CAST(GROUP_CONCAT(id) AS CHAR) AS ids,
SUM(booked) AS booked
FROM
available_dates
WHERE
(start_date>=20120714 AND end_date<=DATE_ADD(20120714, INTERVAL 14 DAY))
GROUP BY
room_id
HAVING
end_date_max=DATE_ADD(20120714, INTERVAL 14 DAY)
This gets me part of the way, however doesn't get me the consecutive results - that is the important part. It also only returns a single result (probably because of the HAVING clause) when I widen the test data.
Can anyone point me in the right direction?

Answer 1

If you have a calendar or a numbers table:
CREATE TABLE num
( i INT NOT NULL
, PRIMARY KEY (i)
) ;
INSERT INTO num
(i)
VALUES
(0), (1), (2), ..., (1000) ;
You could use something like this:
SELECT
avail.room_id,
MIN(avail.start_date) AS start_date_min,
MAX(avail.end_date) AS end_date_max,
CAST(GROUP_CONCAT(avail.id) AS CHAR) AS ids,
SUM(avail.booked) AS booked
FROM
available_dates AS avail
CROSS JOIN
( SELECT DATE('2012-07-14') AS start_date_check
, 52 AS max_week_check
) AS param
JOIN
num
ON avail.start_date = param.start_date_check + INTERVAL num.i WEEK
AND num.i < param.max_week_check
WHERE
avail.booked = 0
GROUP BY
avail.room_id,
( num.i / 2 )
HAVING
COUNT(*) = 2
You could also have this:
WHERE
1 =1 --- no WHERE condition
GROUP BY
avail.room_id,
( num.i / 2 )
HAVING --- and optionally
SUM(avail.booked) = 0 --- this