title: MySQL foreign key to allow NULL?
body: <p>I'm piecing together an image website. The basic schema's pretty simple MySQL, but I'm having some trouble trying to represent possible admin flags associated with an image ("inappropriate", "copyrighted", etc.). My current notion is as follows:</p>
  
  <pre><code>tblImages (
      imageID INT UNSIGNED NOT NULL AUTO_INCREMENT,
      ...
  );
  
  tblImageFlags (
      imageFlagID INT UNSIGNED NOT NULL AUTO_INCREMENT,
      imageID INT UNSIGNED NOT NULL,
      flagTypeID INT UNSIGNED NOT NULL,
      resolutionTypeID INT UNSIGNED NOT NULL,
      ...
  );
  
  luResolutionTypes (
      resolutionTypeID INT UNSIGNED NOT NULL AUTO_INCREMENT,
      resolutionType VARCHAR(63) NOT NULL,
      ...
  );
  </code></pre>
  
  <p>(truncated for ease of reading; assorted foreign keys and indexes are in order, I swear)</p>
  
  <p><code>tblImageFlags.flagTypeID</code> is foreign-keyed on a lookup table of flag types, and as you can imagine <code>tblImageFlags.resolutionTypeID</code> <em>should</em> be foreign-keyed on <code>luResolutionTypes.resolutionTypeID</code>. The issue at hand is that, when a flag is first issued, there is no logical resolution type (I'd declare this a good use of <code>NULL</code>); however, if a value is set, it should be foreign-keyed to the lookup table.</p>
  
  <p>I can't find a MySQL syntax workaround to this situation. Does it exist? The best runners up are:</p>
  
  <ul>
  <li>Add an "unmoderated" resolution type</li>
  <li>Add a <code>NULL</code> entry to <code>luResolutionTypes.resolutionTypeID</code> (would this even work in an <code>AUTO_INCREMENT</code> column?)</li>
  </ul>
  
  <p>Thanks for the insight!</p>
  
  <p>PS Bonus points to whomever tells me whether, in the case of databases, it's "indexes" or "indices".</p>
  
  <hr>
  
  <p><strong>Follow-up</strong>: thanks to Bill Karwin for pointing out what turned out to be a syntax error in the table structure (don't set a column to <code>NOT NULL</code> if you want it to allow <code>NULL</code>!). And once I have enough karma to give you those bonus points, I will :)</p>