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    <p>The problem description and the comment to one of the suggestions gives the business rules:</p> <ul> <li>querying flights with an effective date should return only one record per flight</li> <li>the record returned should be the latest record</li> <li>previous schedules must be kept in the table</li> </ul> <p>The key to the answer is how to determine which is the latest record - the simplest answer would be to add a column that records the timestamp when the row is inserted. Now when you query for a flight and a given effective date, you just get the result with the latest inserted timestamp (which can be done using ORDER BY DESC and take the first row returned).</p> <p>You can do a similar query with just the effective date and return all flights - again, for each flight you just want to return the row that includes the effective date, but with the largest timestamp. There's a neat trick for finding the maximum of something on a group-by-group basis - left join the results with themselves so that left &lt; right, then the maximum is the left value where the right is null. <a href="http://www.xaprb.com/blog/2007/03/14/how-to-find-the-max-row-per-group-in-sql-without-subqueries/" rel="nofollow noreferrer">The author of High Performance MySQL gives a simple example of this.</a></p> <p>It's much easier than trying to retroactively correct the older schedules - and, by the sound of things, the older schedules have to be kept intact to satisfy your business requirements. It also means you can answer historical questions - you can always find out what your schedule table used to look like on a given date - which means it's very handy when generating reports such as "This month's schedule changes" and so on.</p>
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