Get records from one table excluding records from another
<p>I'm trying to select records from the <code>master table</code> where they match records in the <code>new table</code>, but excluding records where the <code>new table</code> matches the <code>old table</code>. The <code>urn</code> field is a common denominator between them all.</p>

<p>My query is this:</p>

<pre><code>SELECT *
FROM `master`
JOIN `new` ON `master`.`urn` = `new`.`urn`
LEFT JOIN `old` ON `old`.`urn` = `new`.`urn`
</code></pre>

<p>I'm sure this should work, but it doesn't return the correct amount of results.</p>

<p>Any suggestions very welcome!</p>