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POMysql limit rows per group weird results
 

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    <p>I wanted to get the latest 4 dates for each <code>symbolid</code>. I adapted the code <a href="http://www.xaprb.com/blog/2006/12/07/how-to-select-the-firstleastmax-row-per-group-in-sql/" rel="nofollow noreferrer">here</a> as follows:</p> <pre><code>set @num := 0, @symbolid := ''; select symbolid, date, @num := if(@symbolid = symbolid, @num + 1, 1) as row_number, @symbolid := symbolid as dummy from projections group by symbolid, date desc having row_number &lt; 5 </code></pre> <p>and get the following results:</p> <pre><code>symbolid date row_number dummy 1 '2011-09-01 00:00:00' 1 1 1 '2011-08-31 00:00:00' 3 1 1 '2011-08-30 00:00:00' 5 1 2 '2011-09-01 00:00:00' 1 2 2 '2011-08-31 00:00:00' 3 2 2 '2011-08-30 00:00:00' 5 2 3 '2011-09-01 00:00:00' 1 3 3 '2011-08-31 00:00:00' 3 3 3 '2011-08-30 00:00:00' 5 3 4 '2011-09-01 00:00:00' 1 4 ... </code></pre> <p>The obvious question is, why did I only get 3 rows per <code>symbolid</code>, and why are they numbered 1,3,5? A few details:</p> <ol> <li>I tried both forcing an index and not (as seen here), and got the same results both ways. </li> <li>The dates are correct, i.e., the listing correctly shows the top 3 dates per <code>symbolid</code>, but the row_number value is off</li> <li>When I don't use the "having" statement, the row numbers are correct, i.e., the most recent date is 1, the next most recent is 2, etc</li> </ol> <p>Obviously the <code>row_number</code> computed field is being affected by the "having" clause, but I don't know how to fix it.</p> <p>I realize that I could just change the "having" to "having row_number &lt; 7" (6 gives the same as 5), but it's very ugly and would like to know what to do to make it "behave".</p>
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