Converting subselects to joins, part 2

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

@@ -0,0 +1,35 @@
+<!--# set var="title" value="Converting subselects to joins, part 2" -->
+<!--# set var="date" value="November 29, 2011" -->
+
+<!--# include file="include/top.html" -->
+
+<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>
+
+<!--# include file="include/bottom.html" -->

index.html

@@ -20,6 +20,7 @@
 <li>2016-Feb-15: <a href="2016-02-15-cable-modem-channel-party.html">Cable modem channel party</a></li>
 <li>2016-Feb-01: <a href="2016-02-01-how-to-enrage-your-cable-modem.html">How to enrage your cable modem</a></li>
 <li>2016-Feb-01: <a href="2016-02-01-hall-of-2-4-ghz-shame-2016-edition.html">Hall of 2.4 GHz Shame, 2016 Edition</a></li>
+<li>2011-Nov-29: <a href="2011-11-29-converting-subselects-to-joins-part-2.html">Converting subselects to joins, part 2</a></li>
 <li>2011-Nov-29: <a href="2011-11-29-safer-data-changes.html">Safe(r) data changes</a></li>
 <li>2011-Aug-09: <a href="2011-08-09-innodb-as-the-default-table-type.html">InnoDB as the default table type</a></li>
 <li>2011-Aug-08: <a href="2011-08-08-database-best-practices-for-future-scalability.html">Database best practices for future scalability</a></li>

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

@@ -0,0 +1,33 @@
+<!--# set var="title" value="Converting subselects to joins, part 2" -->
+<!--# set var="date" value="November 29, 2011" -->
+
+<!--# include file="include/top.html" -->
+
+[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;
+
+<!--# include file="include/bottom.html" -->

markdown/index.md

@@ -19,6 +19,7 @@
 1. 2016-Feb-15: [Cable modem channel party](2016-02-15-cable-modem-channel-party.html)
 1. 2016-Feb-01: [How to enrage your cable modem](2016-02-01-how-to-enrage-your-cable-modem.html)
 1. 2016-Feb-01: [Hall of 2.4 GHz Shame, 2016 Edition](2016-02-01-hall-of-2-4-ghz-shame-2016-edition.html)
+1. 2011-Nov-29: [Converting subselects to joins, part 2](2011-11-29-converting-subselects-to-joins-part-2.html)
 1. 2011-Nov-29: [Safe(r) data changes](2011-11-29-safer-data-changes.html)
 1. 2011-Aug-09: [InnoDB as the default table type](2011-08-09-innodb-as-the-default-table-type.html)
 1. 2011-Aug-08: [Database best practices for future scalability](2011-08-08-database-best-practices-for-future-scalability.html)