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    <p>Both your second and third examples are valid. Here are a few points:</p> <ol> <li>In your second query, it is not neccessary to use <code>DISTINCT</code> that will simply add overhead. When you perform an <code>IN</code> operation, SQL will typically not perform the complete join operation and exit early as soon as a match is found. It also doesn't return all the rows, just true/false whether there is a match for a given value. </li> <li>Using <code>IN</code> in your second example may yield a more optimal join operator (semi-join vs join) because you're explicitly stating that you are not interested in the output from the subquery, just whether or not there are records returned. </li> <li>You may be better off using the <code>EXISTS</code> clause. Although it's a common misconception that <code>IN</code> less efficient than <code>EXISTS</code> (they actually implement queries the same in most cases) <code>IN</code> can give unexpected results when dealing with nulls. </li> </ol> <p>The <code>EXISTS</code> version would look something like this:</p> <pre><code>UPDATE FT SET Flag = 1 FROM FeedbackTable WHERE EXISTS(SELECT * FROM FeedbackIndexed FI INNER JOIN QueueTable QT ON FI.Label = QT.Label AND FI.AccountID = QT.AccountID WHERE FeedbackID = FT.feedbackID) </code></pre> <p>The underlying query plan will likely be exactly the same as your <code>IN</code> example (after you remove the redundant <code>DISTINCT</code>) and it may yield the same query plan as the 3rd example but it's always good to know different approaches to solving a problem. </p> <p>A few more points. </p> <ul> <li>Your <code>TABLOCK</code> will be released when the query completes unless you wrap the query and the query to drop the processed records in an explicit transaction. I'm <em>pretty sure</em> you'll want to add <code>HOLDLOCK</code> here too. <code>HOLDLOCK</code> will hold the lock for the duration of the transaction. </li> <li>You mentioned a concern preventing writes to your queue table, what about reads? <code>TABLOCK</code> will implement a shared lock which may cause a race condition if your consumer proc is running multiple instances simultaneously. Consider using <code>TABLOCKX</code> if that will be a problem. </li> <li>There may be better ways manage locking and race conditions there. I like to use SP_GETAPPLOCK for this. I wrote an article on the subject several years ago, it may be worth a read <a href="http://www.sqlservercentral.com/articles/Miscellaneous/2649/" rel="nofollow">sp_getapplock</a></li> </ul> <p>I hope this helps.</p>
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    1. CO"it is not neccessary to use DISTINCT that will simply add overhead" -- I am mildly surprised the optimizer doesn't just ignore it.
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    2. COIt may very well ignore it these days. I haven't tested that behavior in a very long time.
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    3. CO@magicmike thanks for a good explanation, so you are saying the EXISTS query will be more efficient than an INNER Join? I thought JOINS are usually better than IN or EXISTS ?
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