Converting subselects to joins, part 2

markdown/2011-11-29-converting-subselects-to-joins-part-2.md

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+<!--# set var="title" value="Converting subselects to joins, part 2" -->
+<!--# set var="date" value="November 29, 2011" -->
+
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+
+[I previously discussed this in depth](2011-07-12-converting-subselects-to-joins.html). However, today I saw a case that I didn't cover:
+
+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:
+
+    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;
+
+We can convert this to a simple JOIN with the magic of HAVING. HAVING is like WHERE, but applies after aggregation:
+
+    SELECT
+        UserId,
+        DATEDIFF(NOW(), MAX(TimeStamp)) AS LastLogin
+      FROM Users
+        JOIN Logins USING (UserId)
+      GROUP BY UserId
+      HAVING LastLogin &lt;= 31;
+
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