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copied!<p><code>Group By X</code> means <strong>put all those with the same value for X in the one group</strong>.</p> <p><code>Group By X, Y</code> means <strong>put all those with the same values for both X and Y in the one group</strong>.</p> <p>To illustrate using an example, let's say we have the following table, to do with who is attending what subject at a university:</p> <pre><code>Table: Subject_Selection Subject Semester Attendee --------------------------------- ITB001 1 John ITB001 1 Bob ITB001 1 Mickey ITB001 2 Jenny ITB001 2 James MKB114 1 John MKB114 1 Erica </code></pre> <p>When you use a <code>group by</code> on the subject column only; say:</p> <pre><code>select Subject, Count(*) from Subject_Selection group by Subject </code></pre> <p>You will get something like:</p> <pre><code>Subject Count ------------------------------ ITB001 5 MKB114 2 </code></pre> <p>...because there are 5 entries for ITB001, and 2 for MKB114</p> <p>If we were to <code>group by</code> two columns:</p> <pre><code>select Subject, Semester, Count(*) from Subject_Selection group by Subject, Semester </code></pre> <p>we would get this:</p> <pre><code>Subject Semester Count ------------------------------ ITB001 1 3 ITB001 2 2 MKB114 1 2 </code></pre> <p>This is because, when we group by two columns, it is saying <strong>"Group them so that all of those with the same Subject and Semester are in the same group, and then calculate all the aggregate functions</strong> (Count, Sum, Average, etc.) <strong>for each of those groups"</strong>. In this example, this is demonstrated by the fact that, when we count them, there are <strong>three</strong> people doing ITB001 in semester 1, and <strong>two</strong> doing it in semester 2. Both of the people doing MKB114 are in semester 1, so there is no row for semester 2 (no data fits into the group "MKB114, Semester 2")</p> <p>Hopefully that makes sense.</p>

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