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    <p>Option 3, though it may feel strange, is the closest to what you want to model. You'd do something like:</p> <pre><code>User { UserId PRIMARY KEY (UserId) } Computer { ComputerId, PrimaryUserId PRIMARY KEY (UserId) FOREIGN KEY (ComputerId, PrimaryUserId) REFERENCES Computer_User (ComputerId, UserId) } Computer_User { ComputerId, UserId PRIMARY KEY (ComputerId, UserId) FOREIGN KEY (ComputerId) REFERENCES Computer (ComputerId) FOREIGN KEY (UserId) REFERENCES User (UserId) } </code></pre> <p>Which gives you 0 or 1 primary user (the PrimaryUserId can be nullable if you want), that must be in Computer_User. Edit: If a user can only be primary for 1 computer, then a UNIQUE CONSTRAINT on Computer.PrimaryUserId will enforce that. Note that there is no requirement that all users be a primary on some computer (that would be a 1:1 relationship, and would call for them to be in the same table).</p> <p>Edit: Some queries to show you the simplicity of this design</p> <pre><code>--All users of a computer SELECT User.* FROM User JOIN Computer_User ON User.UserId = Computer_User.UserId WHERE Computer_User.ComputerId = @computerId --Primary user of a computer SELECT User.* FROM User JOIN Computer ON User.UserId = Computer.PrimaryUserId WHERE Computer.ComputerId = @computerId --All computers a user has access to SELECT Computer.* FROM Computer JOIN Computer_User ON Computer.ComputerId = Computer_User.ComputerId WHERE Computer_User.UserId = @userId --Primary computer for a user SELECT Computer.* FROM Computer WHERE PrimaryUserId = @userId </code></pre>
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    1. USMark Brackett
    1. USMark Brackett
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    1. COThe FK Computer -> CompUser enforces that the primary user for a computer is indeed one of that computer's users. Strictness is good :)
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      1. UShelios
    2. COThanks for the post. It got me looking at the relationship in a way that initially seems foreign, but really works well.
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      1. USNoah Goodrich
    3. COI liked it! On a theorical point of view, we are saying here that we have a "one-to-many" relationship called "is the primary user of computer ..." in addition to the "many-to-many" relationship called "is a user of computer". Did someone see any limitation to this proposal?
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      1. USPhilippe Grondier
 

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