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copied!<p><a href="http://en.wikipedia.org/wiki/Foreign_key" rel="nofollow noreferrer">Foreign keys</a> will guarantee that a row in a table <code>order_details</code> with a field <code>order_id</code> referencing an <code>orders</code> table will never have an <code>order_id</code> value that doesn't exist in the <code>orders</code> table. </p> <p>Foreign keys aren't required to have a working relational database, but they are definitely essential to avoid broken relationships and orphan rows (ie. <a href="http://en.wikipedia.org/wiki/Referential_integrity" rel="nofollow noreferrer">referential integrity</a>). The ability to enforce referential integrity at the database level is required for the <em>C</em> in <a href="http://en.wikipedia.org/wiki/ACID" rel="nofollow noreferrer">ACID</a> to stand.</p> <p>As for your concerns regarding performance, in general the performance hit, if any, will be negligible. I suggest putting in all your foreign key constraints, and only experiment without them if you have real performance issues that you cannot solve otherwise.</p> <p>In addition, as a side-note, while not directly related to MySQL, this is quote from <a href="http://msdn.microsoft.com/en-us/library/ms998577.aspx" rel="nofollow noreferrer">Microsoft Patterns and Practices: Chapter 14 Improving SQL Server Performance</a>:</p> <blockquote> <p>When primary and foreign keys are defined as constraints in the database schema, the server can use that information to create optimal execution plans.</p> </blockquote>

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