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    <p>Here's a solution that seems to work:</p> <pre><code>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 &gt;= b1.End) -- edit: new term ) AS t LEFT OUTER JOIN LettingUnitBookings x ON (t.ID = x.F_LU_ID AND x.Start &lt; t.LatestAvailable AND x.End &gt; t.EarliestAvailable) WHERE x.ID IS NULL AND DATEDIFF(t.LatestAvailable, t.EarliestAvailable) &gt;= @WindowSize; </code></pre> <p>The output is:</p> <pre><code>+-----+-------------+-------------------+-----------------+-----------------+ | 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 | +-----+-------------+-------------------+-----------------+-----------------+ </code></pre> <p>Analyzing this with <code>EXPLAIN</code> shows that it employs indexes pretty well:</p> <pre><code>+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+ | 1 | PRIMARY | &lt;derived2&gt; | 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 | | +----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------------------+ </code></pre> <p>Compare with the <code>EXPLAIN</code> report for the solution <a href="https://stackoverflow.com/questions/1687519/searching-for-availability-with-mysql-and-php/1691282#1691282">given</a> by @martin clayton:</p> <pre><code>+----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+ | 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 | &lt;derived2&gt; | ALL | NULL | NULL | NULL | NULL | 4 | Using where | | 2 | DERIVED | &lt;derived3&gt; | ALL | NULL | NULL | NULL | NULL | 4 | Using temporary; Using filesort | | 2 | DERIVED | &lt;derived5&gt; | 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 | &lt;union5,6&gt; | 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 | &lt;union3,4&gt; | ALL | NULL | NULL | NULL | NULL | NULL | | +----+--------------+---------------------+--------+---------------+---------+---------+------+------+---------------------------------+ </code></pre> <p>In general, you want to avoid optimization plans that force <code>Using filesort</code> or <code>Using temporary</code> because these are performance killers. A query using <code>GROUP BY</code> is almost certain to cause this kind of optimization, at least in MySQL.</p>
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    1. CO@Bill - nice optimise - I put an extra LettingUnit with bookings 2009/01/01-2009/01/20 and 2009/02/25-2009/02/28 in my test dataset, but its not appearing in the result set using your query. Will take a look if I have a moment later.
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    2. CO@martin: I've added a term to the query to handle this. It hasn't changed the optimization plan.
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