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    <blockquote> <p>Since Primary Keys are UNIQUE by default, this makes this relation One to One.</p> </blockquote> <p>No, that makes the relation "one to zero or one". Is that what you actually need?</p> <p>If <strong>yes</strong>, then then your "second solution" is better:</p> <ul> <li>it's simpler,</li> <li>takes less storage<sup>1</sup> (and therefore makes cache "larger")</li> <li>hes less indexes to maintain<sup>2</sup>, which benefits data manipulation,</li> <li>and (since you are using InnoDB) naturally <a href="http://use-the-index-luke.com/sql/clustering/index-organized-clustered-index" rel="noreferrer">clusters</a> the data, so users that are close together will have their accounts stored close together as well, which may benefit cache locality and certain kinds of range scans.</li> </ul> <p>BTW, you'll need to make <code>accounts.id</code> an ordinary integer (not auto-increment) for this to work.</p> <p>If <strong>no</strong>, see below...</p> <blockquote> <p>What is the best way to create One to One relation in MySQL?</p> </blockquote> <p>Well, "best" is an overloaded word, but the "standard" solution would be the same as in any other database: put both entities (user and account in your case) in the same physical table.</p> <blockquote> <p>Are there any other solutions other than these two?</p> </blockquote> <p>Theoretically, you could make circular FKs between the two PKs, but that would require <em>deferred</em> constraints to resolve the chicken-and-egg problem, which are unfortunately not supported under MySQL.</p> <blockquote> <p>And if I import any of these solutions into MySQL Workbench EER diagram, it recognizes relations as One to Many :S Thats also confusing.</p> </blockquote> <p>I don't have much practical experience with that particular modeling tool, but I'm guessing that's because it is "one to many" where "many" side was capped at 1 by making it unique. Please remember that "many" doesn't mean "1 or many", it means "0 or many", so the "capped" version really means "0 or 1".</p> <hr> <p><em><sup>1</sup> Not just in the storage expense for the additional field, but for the secondary index as well. And since you are using InnoDB which <a href="http://www.ovaistariq.net/521/understanding-innodb-clustered-indexes/" rel="noreferrer">always clusters tables</a>, beware that secondary indexes are even more expensive in clustered tables than they are in heap-based tables.</em></p> <p><em><sup>2</sup> InnoDB <a href="http://dev.mysql.com/doc/refman/5.6/en/innodb-foreign-key-constraints.html" rel="noreferrer">requires indexes</a> on foreign keys.</em></p>
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    1. CO@Limeni My pleasure! BTW, this is an English site, and using your (in this case our) local language is not recommended.
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