Converting subselects to joins, part 2

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

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+<!--# 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 &amp;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 &amp;lt;= 31;
+</code></pre>
+
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