Searching for availability with MySQL (and PHP)?

Question

I have two MySQL (MyIsAm) tables that represent letting units and bookings:

• LettingUnits (ID, Name, etc...)
• LettingUnitBooki

Answer 1

It's not pretty.

• Join LettingUnitBookings to itself
• Find the start and end of gaps between bookings for each F_LU_ID
• Get the size of the gaps - the available 'slots'
• Consider the case where there are no existing bookings that 'bracket' a suitable slot, add in outlier dates for this
• Join that projection to the LettingUnits table and apply WHERE criteria (start, end, duration)

I've neglected to include BookingUnits that have no bookings at all.

Ends up looking like this:

SELECT @StartOfWindow := '2009-01-01',
       @EndOfWindow   := '2009-02-01',
       @WindowSize    := 5
;

SELECT
    lu.Name,
    Slots.*
FROM (
    SELECT
        lub1.F_LU_ID,
        DATE_ADD( MAX( lub2.date_time ), INTERVAL 1 DAY )     AS StartOfSlot,
        DATE_SUB( lub1.date_time, INTERVAL 1 DAY )            AS EndOfSlot,
        DATEDIFF( lub1.date_time, MAX( lub2.date_time ) ) - 1 AS AvailableDays
    FROM
    ( SELECT F_LU_ID, Start AS date_time FROM LettingUnitBookings
      UNION
      SELECT F_LU_ID, CAST( '9999-12-31' AS DATE ) FROM LettingUnitBookings
    ) AS lub1,
    ( SELECT F_LU_ID, End   AS date_time FROM LettingUnitBookings
      UNION
      SELECT F_LU_ID, CAST( '1000-01-01' AS DATE ) FROM LettingUnitBookings
    ) AS lub2
    WHERE
        lub2.date_time <= lub1.date_time
    AND lub2.F_LU_ID = lub1.F_LU_ID
    GROUP BY
        lub1.F_LU_ID,
        lub1.date_time
    ) Slots
JOIN LettingUnits lu
ON   lu.ID = Slots.F_LU_ID
WHERE
    Slots.AvailableDays >= @WindowSize
AND (
   (     DATEDIFF( Slots.EndOfSlot, @EndOfWindow )     >= @WindowSize
     AND DATEDIFF( @StartOfWindow, Slots.StartOfSlot ) >= @WindowSize
   )
   OR
   (     DATEDIFF( @EndOfWindow, Slots.StartOfSlot ) >= @WindowSize
     AND DATEDIFF( Slots.EndOfSlot, @StartOfWindow ) >= @WindowSize
   )
)

Gives

Name        F_LU_ID StartOfSlot EndOfSlot  AvailableDays
Foo Cottage 123     2009-01-06  2009-01-09 5
Foo Cottage 123     2009-01-21  2009-01-24 5

Hopefully that can be adapted to suit your needs.

Alternatively, if a booking can start on the same day that the previous booking ends, you can adapt slightly...

SELECT
    lu.Name,
    Slots.*
FROM (
    SELECT
        lub1.F_LU_ID,
        MAX( lub2.date_time ) AS StartOfSlot,
        lub1.date_time        AS EndOfSlot,
        DATEDIFF( lub1.date_time, MAX( lub2.date_time )) AS AvailableDays
    FROM
    ( SELECT F_LU_ID, Start AS date_time FROM LettingUnitBookings
      UNION
      SELECT F_LU_ID, CAST( '9999-12-31' AS DATE ) FROM LettingUnitBookings
    ) AS lub1,
    ( SELECT F_LU_ID, End   AS date_time FROM LettingUnitBookings
      UNION
      SELECT F_LU_ID, CAST( '1000-01-01' AS DATE ) FROM LettingUnitBookings
    ) AS lub2
    WHERE
        lub2.date_time <= lub1.date_time
    AND lub2.F_LU_ID = lub1.F_LU_ID
    GROUP BY
        lub1.F_LU_ID,
        lub1.date_time
    ) Slots
JOIN LettingUnits lu
ON   lu.ID = Slots.F_LU_ID
WHERE
    Slots.AvailableDays >= @WindowSize
AND
   (     DATEDIFF( Slots.EndOfSlot, @EndOfWindow )     >= @WindowSize
     AND DATEDIFF( @StartOfWindow, Slots.StartOfSlot ) >= @WindowSize
   )
   OR
   (     DATEDIFF( @EndOfWindow, Slots.StartOfSlot ) >= @WindowSize
     AND DATEDIFF( Slots.EndOfSlot, @StartOfWindow ) >= @WindowSize
   )

Answer 2

Here's a solution that seems to work:

SELECT t.*, DATEDIFF(t.LatestAvailable, t.EarliestAvailable) AS LengthAvailable
FROM 
   (SELECT u.*,
      COALESCE(b1.End, @StartOfWindow) AS EarliestAvailable,
      COALESCE(b2.Start, @EndOfWindow) AS LatestAvailable
    FROM LettingUnits u
    LEFT OUTER JOIN LettingUnitBookings b1
      ON (u.ID = b1.F_LU_ID AND b1.End BETWEEN @StartOfWindow AND @EndOfWindow)
    LEFT OUTER JOIN LettingUnitBookings b2
      ON (u.ID = b2.F_LU_ID AND b2.Start BETWEEN @StartOfWindow AND @EndOfWindow
          AND b2.Start >= b1.End) -- edit: new term
    ) AS t
LEFT OUTER JOIN LettingUnitBookings x
  ON (t.ID = x.F_LU_ID AND x.Start < t.LatestAvailable AND x.End > t.EarliestAvailable)
WHERE x.ID IS NULL AND DATEDIFF(t.LatestAvailable, t.EarliestAvailable) >= @WindowSize;

The output is:

+-----+-------------+-------------------+-----------------+-----------------+
| ID  | Name        | EarliestAvailable | LatestAvailable | LengthAvailable |
+-----+-------------+-------------------+-----------------+-----------------+
| 123 | Foo Cottage | 2009-01-05        | 2009-01-10      |               5 |
| 123 | Foo Cottage | 2009-01-20        | 2009-01-25      |               5 |
| 456 | Bar Cottage | 2009-01-20        | 2009-01-31      |              11 |
+-----+-------------+-------------------+-----------------+-----------------+

Analyzing this with EXPLAIN shows that it employs indexes pretty well:

+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra                   |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+
|  1 | PRIMARY     | <derived2> | ALL    | NULL          | NULL    | NULL    | NULL  |    9 | Using where             |
|  1 | PRIMARY     | x          | ref    | F_LU_ID       | F_LU_ID | 8       | t.ID  |    2 | Using where; Not exists |
|  2 | DERIVED     | u          | system | NULL          | NULL    | NULL    | NULL  |    1 |                         |
|  2 | DERIVED     | b1         | ref    | F_LU_ID       | F_LU_ID | 8       | const |    0 |                         |
|  2 | DERIVED     | b2         | ref    | F_LU_ID       | F_LU_ID | 8       | const |    0 |                         |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+

Compare with the EXPLAIN report for the solution given by @martin clayton:

+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
| id | select_type  | table               | type   | possible_keys | key     | key_len | ref  | rows | Extra                           |
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+
|  1 | PRIMARY      | lu                  | system | PRIMARY,ID    | NULL    | NULL    | NULL |    1 |                                 |
|  1 | PRIMARY      | <derived2>          | ALL    | NULL          | NULL    | NULL    | NULL |    4 | Using where                     |
|  2 | DERIVED      | <derived3>          | ALL    | NULL          | NULL    | NULL    | NULL |    4 | Using temporary; Using filesort |
|  2 | DERIVED      | <derived5>          | ALL    | NULL          | NULL    | NULL    | NULL |    4 | Using where; Using join buffer  |
|  5 | DERIVED      | LettingUnitBookings | ALL    | NULL          | NULL    | NULL    | NULL |    3 |                                 |
|  6 | UNION        | LettingUnitBookings | index  | NULL          | F_LU_ID | 8       | NULL |    3 | Using index                     |
| NULL | UNION RESULT | <union5,6>          | ALL    | NULL          | NULL    | NULL    | NULL | NULL |                                 |
|  3 | DERIVED      | LettingUnitBookings | ALL    | NULL          | NULL    | NULL    | NULL |    3 |                                 |
|  4 | UNION        | LettingUnitBookings | index  | NULL          | F_LU_ID | 8       | NULL |    3 | Using index                     |
| NULL | UNION RESULT | <union3,4>          | ALL    | NULL          | NULL    | NULL    | NULL | NULL |                                 |
+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+

In general, you want to avoid optimization plans that force Using filesort or Using temporary because these are performance killers. A query using GROUP BY is almost certain to cause this kind of optimization, at least in MySQL.