36 lines
1.1 KiB
HTML
36 lines
1.1 KiB
HTML
<!--# set var="title" value="Converting subselects to joins, part 2" -->
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<!--# set var="date" value="November 29, 2011" -->
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<!--# include file="include/top.html" -->
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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>
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<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>
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<pre><code>SELECT
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UserId,
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LastLogin
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FROM Users
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JOIN (
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SELECT
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UserId,
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DATEDIFF(NOW(), MAX(TimeStamp)) AS LastLogin
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FROM Logins
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GROUP BY UserId
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) AS Temp USING (UserId)
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WHERE LastLogin <= 31;
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</code></pre>
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<p>We can convert this to a simple JOIN with the magic of HAVING. HAVING is like WHERE, but applies after aggregation:</p>
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<pre><code>SELECT
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UserId,
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DATEDIFF(NOW(), MAX(TimeStamp)) AS LastLogin
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FROM Users
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JOIN Logins USING (UserId)
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GROUP BY UserId
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HAVING LastLogin <= 31;
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</code></pre>
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