Question

I’m making a booking system in PHP/MySQL.

I want to be able to select a month, select a booking length, and find how many periods of the given length are available (ie, not marked as booked) within that month.

I started by creating a table of days, each day has a field that indicates if it’s reserved or not, as well as a date. The two tables are:

CREATE TABLE `day` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `date` date NOT NULL,
  `booking` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) 

CREATE TABLE `booking` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `start` date DEFAULT NULL,
  `end` date DEFAULT NULL,
  PRIMARY KEY (`id`)
) 

So far I’ve been able to count how many days in a month aren’t booked – but that doesn’t necessarily tell me if say there are 7 days in a row:

SELECT  
    EXTRACT(YEAR FROM date) as year,
    EXTRACT( MONTH FROM date) as month,
    count(day.id)
FROM
    day
WHERE booking IS NULL
GROUP BY year, month

At this point I’m considering loading all the days for my given month, and iterating through them in PHP to make up my numbers, but it seems kinda dumb and inefficient. Does anyone have a suggestion how I can find this information directly from MySQL?

Answer 1

Assuming you want ranges of at least 7 days in July, 2012, then:

SELECT
  DATE_FORMAT(start, '%Y-%m-%d') start,
  MAX(days) AS days,
  DATE_FORMAT(start + INTERVAL MAX(days) - 1 DAY, '%Y-%m-%d') end
FROM (  
  SELECT
    d2.`date` start,
    (@days := IF(IFNULL(d2.booking, 0) = 0, @days + 1, 1)) AS days
  FROM
    (SELECT @days := 1) a,
    day d1
  LEFT JOIN  
    day d2 on d2.`date` = d1.`date` + INTERVAL 1 DAY
  WHERE
    IFNULL(d1.booking, 0) = 0
) d1
GROUP BY
  start
HAVING
  (
    EXTRACT(YEAR_MONTH FROM start) = 201207 OR
    EXTRACT(YEAR_MONTH FROM (start + INTERVAL MAX(days) - 1 DAY)) = 201207
  ) AND
  days >= 7
ORDER BY
  start

will produce this output:

|      START | DAYS |        END |
----------------------------------
| 2012-06-25 |   13 | 2012-07-07 |
| 2012-06-26 |   12 | 2012-07-07 |
| 2012-06-27 |   11 | 2012-07-07 |
| 2012-06-28 |   10 | 2012-07-07 |
| 2012-06-29 |    9 | 2012-07-07 |
| 2012-06-30 |    8 | 2012-07-07 |
| 2012-07-01 |    7 | 2012-07-07 |
| 2012-07-23 |    8 | 2012-07-30 |
| 2012-07-24 |    7 | 2012-07-30 |

The clause

(  
    EXTRACT(YEAR_MONTH FROM start) = 201207 OR
    EXTRACT(YEAR_MONTH FROM (start + INTERVAL MAX(days) - 1 DAY)) = 201207
) 

is used to include all ranges, even if the range extends into the previous, or next month. If you want ranges that fall exclusively in a single month, use:

EXTRACT(YEAR_MONTH FROM start) = 201207 AND
EXTRACT(YEAR_MONTH FROM (start + INTERVAL MAX(days) - 1 DAY)) = 201207

See http://sqlfiddle.com/#!2/4e36a/4 for an interactive demo.