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PORelational database design - I want an extra field for some row types but not all
 

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  1. PORelational database design - I want an extra field for some row types but not all
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    <p>This a very basic database design/normalisation question. </p> <p>Suppose I have a <code>Books</code> table with the following columns:</p> <pre><code>isbn|title|author|status </code></pre> <p>and <code>status</code> can be one of <code>checked out</code>, <code>available</code>, <code>overdue</code>, <code>lost</code> (stored as integers).</p> <p>When adding rows I decide "actually, when the status is <code>checked out</code>, I want to store another field <code>due_date</code>". <em>I only want to store this field for books with status checked out</em>, as it has no meaning otherwise. </p> <p><strong>What is the standard, correct, canonical way to do this?</strong></p> <p>One approach is to add the column and set it to <code>NULL</code> if the status is not <code>checked out</code>, but this sounds like a bad idea to me (for integrity among other things, e.g. what if the status is <code>available</code> and we also have a <code>due_date</code>?)</p> <p>The other obvious answer is to create a <code>DueDates</code> table and store <code>isbn|due_date</code> pairs in it. This is the approach I normally take but it's easy to end up with tables and <code>JOIN</code>s all over the place. </p> <p>I am not looking for how to store books specifically, that's just an example of the problem and I want to know the standard solution.</p> <p><strong>Edit</strong>: Does the answer change if I decide that I want to add <em>lots</em> of fields for <code>checked out</code> status only (<code>due_date</code>, <code>borrowed_by</code>, <code>checked_out_from</code>, ...) - and have all these as <code>NULL</code> if the status is not <code>checked out</code>?</p>
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    1. COFrom a *practical* standpoint, in e.g. SQL, we can implement a `CHECK` constraint, such that `status = checked out` and `due_date is not null`, or `status != checked out` and `due_date is null`.
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