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    <p>the <code>"WHERE"</code> for <code>GROUP BY</code> is <code>HAVING</code>:</p> <pre><code>SELECT ipaddr from visitors GROUP BY ipaddr HAVING COUNT(ipaddr) = 1 AND MIN(date) &gt; '2011-03-31 00:59:11' </code></pre> <h2>UPDATE</h2> <pre><code>SELECT ipaddr, max(sid) sid FROM visitors GROUP BY ipaddr HAVING COUNT(DISTINCT sid) = 1 AND MIN(date) &gt; '2011-03-31 00:59:11' </code></pre> <p><strong>Explanation:</strong></p> <pre><code>SELECT date, sid, ipaddr FROM visitors date sid ipaddr ------------------------------------------ 2011-03-31 06:25:48 299525457 -1454342140 2011-03-31 06:26:37 299525457 -1454342140 2010-01-01 15:23:44 694388590 -1454342140 2011-03-31 11:23:44 111111111 -1234444811 2011-03-31 12:23:44 111111111 -1234444811 SELECT ipaddr FROM visitors GROUP BY ipaddr ipaddr ----------- -1454342140 -1234444811 --- group for ip -1454342140 --- 2011-03-31 06:25:48 299525457 -1454342140 2011-03-31 06:26:37 299525457 -1454342140 2010-01-01 15:23:44 694388590 -1454342140 COUNT(DISTINCT sid) = COUNT(299525457, 694388590) = 2 --&gt; there is more than 1 session for this ip: not good!!! ==&gt; group discarded --- group for ip -1234444811 --- 2011-03-31 11:23:44 111111111 -1234444811 2011-03-31 12:23:44 111111111 -1234444811 COUNT(DISTINCT sid) = COUNT(111111111) = 1 --&gt; OK (here COUNT(sid) = count(111111111, 111111111) = 2 --&gt; despite it is the same sid, the count is 2, that is why using DISTINCT) MIN(date) = '2011-03-31 11:23:44' &gt; '2011-03-31 00:59:11' --&gt; OK ==&gt; group accepted </code></pre> <p>Authorized columns in the <code>SELECT</code> are:</p> <ul> <li>columns used in the <code>GROUP BY</code> clause</li> <li>agregates of the other columns</li> </ul> <p><code>ipaddr</code> was used in GROUP BY but not <code>sid</code>. To have also <code>sid</code> I used MAX but remember that it will be applied only to the group of rows for current <code>ipaddr</code> and because of the conditions in the query there is 1 unique <code>sid</code> but repeated so the result will be that <code>sid</code></p>
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    1. COHmm this seems to do what I described. Can you please explain how this query works? I thought that GROUP BY is applied before HAVING, therefore GROUP by would leave 1 row per ipaddr. Then, wouldn't count(ipaddr) equal 1 for all the rows left after GROUP BY?
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    2. COthe HAVING clause is used to filter on Aggregates (SUM, COUNT, AVG), the WHERE clause filters on conditions. So it does GROUP BY first and then eliminates all the records that don't meet the HAVING condition.
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    3. COthe `HAVING` clause is used with aggregate functions to filter groups produced by `GROUP BY`, i.e.: before returning `ipaddr`, `count(ipaddr) = 1` is evaluated against the group of rows with that `ipaddr`
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