firestuff/2011-11-29-converting-subselects-to-joins-part-2.html

<!--# set var="title" value="Converting subselects to joins, part 2" -->
<!--# set var="date" value="November 29, 2011" -->

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<p><a href="2011-07-12-converting-subselects-to-joins.html">I previously discussed this in depth</a>. However, today I saw a case that I didn't cover:</p>

<p>You have a table of Users and a table of Logins, with a row for each user login event. You're looking for users that have logged in within the last 31 days. The initial version of this I saw used a derived table:</p>

<pre><code>SELECT
    UserId,
    LastLogin
  FROM Users
    JOIN (
      SELECT
          UserId,
          DATEDIFF(NOW(), MAX(TimeStamp)) AS LastLogin
        FROM Logins
        GROUP BY UserId
    ) AS Temp USING (UserId)
  WHERE LastLogin &lt;= 31;
</code></pre>

<p>We can convert this to a simple JOIN with the magic of HAVING. HAVING is like WHERE, but applies after aggregation:</p>

<pre><code>SELECT
    UserId,
    DATEDIFF(NOW(), MAX(TimeStamp)) AS LastLogin
  FROM Users
    JOIN Logins USING (UserId)
  GROUP BY UserId
  HAVING LastLogin &lt;= 31;
</code></pre>

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