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<h1>Aggregating and Grouping Data</h1>
<p> Last updated on 2023-04-21 |
<a href="https://github.com/datacarpentry/sql-ecology-lesson/edit/main/episodes/02-sql-aggregation.md" class="external-link">Edit this page <i aria-hidden="true" data-feather="edit"></i></a></p>
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<ul><li>How can I summarize my data by aggregating, filtering, or ordering
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<ul><li>Apply aggregation functions to group records together.</li>
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<section id="count-and-group-by"><h2 class="section-heading">COUNT and GROUP BY<a class="anchor" aria-label="anchor" href="#count-and-group-by"></a>
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<hr class="half-width"><p>Aggregation allows us to combine results by grouping records based on
value. It is also useful for calculating combined values in groups.</p>
<p>Let’s go to the surveys table and find out how many individuals there
are. Using the wildcard * counts the number of records (rows):</p>
<div class="codewrapper sourceCode" id="cb1">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>We can also find out how much all of those individuals weigh:</p>
<div class="codewrapper sourceCode" id="cb2">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">COUNT</span>(<span class="op">*</span>), <span class="fu">SUM</span>(weight)</span>
<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>We can output this value in kilograms (dividing the value by
1000.00), then rounding to 3 decimal places: (Notice the divisor has
numbers after the decimal point, which forces the answer to have a
decimal fraction)</p>
<div class="codewrapper sourceCode" id="cb3">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">ROUND</span>(<span class="fu">SUM</span>(weight)<span class="op">/</span><span class="fl">1000.00</span>, <span class="dv">3</span>)</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>There are many other aggregate functions included in SQL, for
example: <code>MAX</code>, <code>MIN</code>, and <code>AVG</code>.</p>
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</h3>
<div class="callout-content">
<p>Write a query that returns: the total weight, average weight, minimum
and maximum weights for all animals caught over the duration of the
survey. Can you modify it so that it outputs these values only for
weights between 5 and 10?</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="co">-- All animals</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">SUM</span>(weight), <span class="fu">AVG</span>(weight), <span class="fu">MIN</span>(weight), <span class="fu">MAX</span>(weight)</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="co">-- Only weights between 5 and 10</span></span>
<span id="cb4-6"><a href="#cb4-6" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">SUM</span>(weight), <span class="fu">AVG</span>(weight), <span class="fu">MIN</span>(weight), <span class="fu">MAX</span>(weight)</span>
<span id="cb4-7"><a href="#cb4-7" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb4-8"><a href="#cb4-8" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> (weight <span class="op">></span> <span class="dv">5</span>) <span class="kw">AND</span> (weight <span class="op"><</span> <span class="dv">10</span>);</span></code></pre>
</div>
</div>
</div>
</div>
</div>
<p>Now, let’s see how many individuals were counted in each species. We
do this using a <code>GROUP BY</code> clause</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> species_id, <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> species_id;</span></code></pre>
</div>
<p><code>GROUP BY</code> tells SQL what field or fields we want to use
to aggregate the data. If we want to group by multiple fields, we give
<code>GROUP BY</code> a comma separated list.</p>
<div id="challenge-1" class="callout challenge">
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<h3 class="callout-title">Challenge<a class="anchor" aria-label="anchor" href="#challenge-1"></a>
</h3>
<div class="callout-content">
<p>Write queries that return:</p>
<ol style="list-style-type: decimal"><li>How many individuals were counted in each year in total</li>
<li>How many were counted each year, for each different species</li>
<li>The average weights of each species in each year</li>
</ol><p>Can you get the answer to both 2 and 3 in a single query?</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="dt">year</span>, <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb6-3"><a href="#cb6-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> <span class="dt">year</span>;</span></code></pre>
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Solution of 2 and 3
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="dt">year</span>, species_id, <span class="fu">COUNT</span>(<span class="op">*</span>), <span class="fu">AVG</span>(weight)</span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> <span class="dt">year</span>, species_id;</span></code></pre>
</div>
</div>
</div>
</div>
</div>
</section><section id="ordering-aggregated-results"><h2 class="section-heading">Ordering Aggregated Results<a class="anchor" aria-label="anchor" href="#ordering-aggregated-results"></a>
</h2>
<hr class="half-width"><p>We can order the results of our aggregation by a specific column,
including the aggregated column. Let’s count the number of individuals
of each species captured, ordered by the count:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb8-1"><a href="#cb8-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> species_id, <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb8-3"><a href="#cb8-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> species_id</span>
<span id="cb8-4"><a href="#cb8-4" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> <span class="fu">COUNT</span>(species_id);</span></code></pre>
</div>
</section><section id="aliases"><h2 class="section-heading">Aliases<a class="anchor" aria-label="anchor" href="#aliases"></a>
</h2>
<hr class="half-width"><p>As queries get more complex, the expressions we use can get long and
unwieldy. To help make things clearer in the query and in its output, we
can use aliases to assign new names to things in the query.</p>
<p>We can use aliases in column names using <code>AS</code>:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb9-1"><a href="#cb9-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">MAX</span>(<span class="dt">year</span>) <span class="kw">AS</span> last_surveyed_year</span>
<span id="cb9-2"><a href="#cb9-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>The <code>AS</code> isn’t technically required, so you could do</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">MAX</span>(<span class="dt">year</span>) last_surveyed_year</span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>but using <code>AS</code> is much clearer so it is good style to
include it.</p>
<p>We can not only alias column names, but also table names in the same
way:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span></span>
<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys <span class="kw">AS</span> surv;</span></code></pre>
</div>
<p>And again, the <code>AS</code> keyword is not required, so this
works, too:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span></span>
<span id="cb12-2"><a href="#cb12-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys surv;</span></code></pre>
</div>
<p>Aliasing table names can be helpful when working with queries that
involve multiple tables; you will learn more about this later.</p>
</section><section id="the-having-keyword"><h2 class="section-heading">The <code>HAVING</code> keyword<a class="anchor" aria-label="anchor" href="#the-having-keyword"></a>
</h2>
<hr class="half-width"><p>In the previous episode, we have seen the keyword <code>WHERE</code>,
allowing to filter the results according to some criteria. SQL offers a
mechanism to filter the results based on <strong>aggregate
functions</strong>, through the <code>HAVING</code> keyword.</p>
<p>For example, we can request to only return information about species
with a count higher than 10:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> species_id, <span class="fu">COUNT</span>(species_id)</span>
<span id="cb13-2"><a href="#cb13-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb13-3"><a href="#cb13-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> species_id</span>
<span id="cb13-4"><a href="#cb13-4" aria-hidden="true" tabindex="-1"></a><span class="kw">HAVING</span> <span class="fu">COUNT</span>(species_id) <span class="op">></span> <span class="dv">10</span>;</span></code></pre>
</div>
<p>The <code>HAVING</code> keyword works exactly like the
<code>WHERE</code> keyword, but uses aggregate functions instead of
database fields to filter.</p>
<p>You can use the <code>AS</code> keyword to assign an alias to a
column or table, and refer to that alias in the <code>HAVING</code>
clause. For example, in the above query, we can call the
<code>COUNT(species_id)</code> by another name, like
<code>occurrences</code>. This can be written this way:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb14-1"><a href="#cb14-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> species_id, <span class="fu">COUNT</span>(species_id) <span class="kw">AS</span> occurrences</span>
<span id="cb14-2"><a href="#cb14-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb14-3"><a href="#cb14-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> species_id</span>
<span id="cb14-4"><a href="#cb14-4" aria-hidden="true" tabindex="-1"></a><span class="kw">HAVING</span> occurrences <span class="op">></span> <span class="dv">10</span>;</span></code></pre>
</div>
<p>Note that in both queries, <code>HAVING</code> comes <em>after</em>
<code>GROUP BY</code>. One way to think about this is: the data are
retrieved (<code>SELECT</code>), which can be filtered
(<code>WHERE</code>), then joined in groups (<code>GROUP BY</code>);
finally, we can filter again based on some of these groups
(<code>HAVING</code>).</p>
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</h3>
<div class="callout-content">
<p>Write a query that returns, from the <code>species</code> table, the
number of <code>species</code> in each <code>taxa</code>, only for the
<code>taxa</code> with more than 10 <code>species</code>.</p>
</div>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb15-1"><a href="#cb15-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> taxa, <span class="fu">COUNT</span>(<span class="op">*</span>) <span class="kw">AS</span> n</span>
<span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> species</span>
<span id="cb15-3"><a href="#cb15-3" aria-hidden="true" tabindex="-1"></a><span class="kw">GROUP</span> <span class="kw">BY</span> taxa</span>
<span id="cb15-4"><a href="#cb15-4" aria-hidden="true" tabindex="-1"></a><span class="kw">HAVING</span> n <span class="op">></span> <span class="dv">10</span>;</span></code></pre>
</div>
</div>
</div>
</div>
</div>
</section><section id="saving-queries-for-future-use"><h2 class="section-heading">Saving Queries for Future Use<a class="anchor" aria-label="anchor" href="#saving-queries-for-future-use"></a>
</h2>
<hr class="half-width"><p>It is not uncommon to repeat the same operation more than once, for
example for monitoring or reporting purposes. SQL comes with a very
powerful mechanism to do this by creating views. Views are a form of
query that is saved in the database, and can be used to look at, filter,
and even update information. One way to think of views is as a table,
that can read, aggregate, and filter information from several places
before showing it to you.</p>
<p>Creating a view from a query requires us to add
<code>CREATE VIEW viewname AS</code> before the query itself. For
example, imagine that our project only covers the data gathered during
the summer (May - September) of 2000. That query would look like:</p>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb16-1"><a href="#cb16-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span></span>
<span id="cb16-2"><a href="#cb16-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb16-3"><a href="#cb16-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> <span class="dt">year</span> <span class="op">=</span> <span class="dv">2000</span> <span class="kw">AND</span> (<span class="dt">month</span> <span class="op">></span> <span class="dv">4</span> <span class="kw">AND</span> <span class="dt">month</span> <span class="op"><</span> <span class="dv">10</span>);</span></code></pre>
</div>
<p>But we don’t want to have to type that every time we want to ask a
question about that particular subset of data. Hence, we can benefit
from a view:</p>
<div class="codewrapper sourceCode" id="cb17">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb17-1"><a href="#cb17-1" aria-hidden="true" tabindex="-1"></a><span class="kw">CREATE</span> <span class="kw">VIEW</span> summer_2000 <span class="kw">AS</span></span>
<span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span></span>
<span id="cb17-3"><a href="#cb17-3" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb17-4"><a href="#cb17-4" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> <span class="dt">year</span> <span class="op">=</span> <span class="dv">2000</span> <span class="kw">AND</span> (<span class="dt">month</span> <span class="op">></span> <span class="dv">4</span> <span class="kw">AND</span> <span class="dt">month</span> <span class="op"><</span> <span class="dv">10</span>);</span></code></pre>
</div>
<p>Using a view we will be able to access these results with a much
shorter notation:</p>
<div class="codewrapper sourceCode" id="cb18">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb18-1"><a href="#cb18-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span></span>
<span id="cb18-2"><a href="#cb18-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb18-3"><a href="#cb18-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> species_id <span class="op">=</span> <span class="st">'PE'</span>;</span></code></pre>
</div>
</section><section id="what-about-null"><h2 class="section-heading">What About NULL?<a class="anchor" aria-label="anchor" href="#what-about-null"></a>
</h2>
<hr class="half-width"><p>From the last example, there should only be five records. If you look
at the <code>weight</code> column, it’s easy to see what the average
weight would be. If we use SQL to find the average weight, SQL behaves
like we would hope, ignoring the NULL values:</p>
<div class="codewrapper sourceCode" id="cb19">
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb19-1"><a href="#cb19-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">AVG</span>(weight)</span>
<span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> species_id <span class="op">=</span> <span class="st">'PE'</span>;</span></code></pre>
</div>
<p>But if we try to be extra clever, and find the average ourselves, we
might get tripped up:</p>
<div class="codewrapper sourceCode" id="cb20">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
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<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb20-1"><a href="#cb20-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">SUM</span>(weight), <span class="fu">COUNT</span>(<span class="op">*</span>), <span class="fu">SUM</span>(weight)<span class="op">/</span><span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb20-2"><a href="#cb20-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb20-3"><a href="#cb20-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> species_id <span class="op">=</span> <span class="st">'PE'</span>;</span></code></pre>
</div>
<p>Here the <code>COUNT</code> function includes all five records (even
those with NULL values), but the <code>SUM</code> only includes the
three records with data in the <code>weight</code> field, giving us an
incorrect average. However, our strategy <em>will</em> work if we modify
the <code>COUNT</code> function slightly:</p>
<div class="codewrapper sourceCode" id="cb21">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb21-1"><a href="#cb21-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">SUM</span>(weight), <span class="fu">COUNT</span>(weight), <span class="fu">SUM</span>(weight)<span class="op">/</span><span class="fu">COUNT</span>(weight)</span>
<span id="cb21-2"><a href="#cb21-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb21-3"><a href="#cb21-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> species_id <span class="op">=</span> <span class="st">'PE'</span>;</span></code></pre>
</div>
<p>When we count the weight field specifically, SQL ignores the records
with data missing in that field. So here is one example where NULLs can
be tricky: <code>COUNT(*)</code> and <code>COUNT(field)</code> can
return different values.</p>
<p>Another case is when we use a “negative” query. Let’s count all the
non-female animals:</p>
<div class="codewrapper sourceCode" id="cb22">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb22-1"><a href="#cb22-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb22-2"><a href="#cb22-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb22-3"><a href="#cb22-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> sex <span class="op">!=</span> <span class="st">'F'</span>;</span></code></pre>
</div>
<p>Now let’s count all the non-male animals:</p>
<div class="codewrapper sourceCode" id="cb23">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb23-1"><a href="#cb23-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb23-2"><a href="#cb23-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb23-3"><a href="#cb23-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> sex <span class="op">!=</span> <span class="st">'M'</span>;</span></code></pre>
</div>
<p>But if we compare those two numbers with the total:</p>
<div class="codewrapper sourceCode" id="cb24">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb24-1"><a href="#cb24-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb24-2"><a href="#cb24-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000;</span></code></pre>
</div>
<p>We’ll see that they don’t add up to the total! That’s because SQL
doesn’t automatically include NULL values in a negative conditional
statement. So if we are querying “not x”, then SQL divides our data into
three categories: ‘x’, ‘not NULL, not x’ and NULL; then, returns the
‘not NULL, not x’ group. Sometimes this may be what we want - but
sometimes we may want the missing values included as well! In that case,
we’d need to change our query to:</p>
<div class="codewrapper sourceCode" id="cb25">
<h3 class="code-label">SQL<i aria-hidden="true" data-feather="chevron-left"></i><i aria-hidden="true" data-feather="chevron-right"></i>
</h3>
<pre class="sourceCode sql" tabindex="0"><code class="sourceCode sql"><span id="cb25-1"><a href="#cb25-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="fu">COUNT</span>(<span class="op">*</span>)</span>
<span id="cb25-2"><a href="#cb25-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> summer_2000</span>
<span id="cb25-3"><a href="#cb25-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> sex <span class="op">!=</span> <span class="st">'M'</span> <span class="kw">OR</span> sex <span class="kw">IS</span> <span class="kw">NULL</span>;</span></code></pre>
</div>
<div id="keypoints1" class="callout keypoints">
<div class="callout-square">
<i class="callout-icon" data-feather="key"></i>
</div>
<div class="callout-inner">
<h3 class="callout-title">Keypoints<a class="anchor" aria-label="anchor" href="#keypoints1"></a>
</h3>
<div class="callout-content">
<ul><li>Use the <code>GROUP BY</code> keyword to aggregate data.</li>
<li>Functions like <code>MIN</code>, <code>MAX</code>, <code>AVG</code>,
<code>SUM</code>, <code>COUNT</code>, etc. operate on aggregated
data.</li>
<li>Aliases can help shorten long queries. To write clear and readable
queries, use the <code>AS</code> keyword when creating aliases.</li>
<li>Use the <code>HAVING</code> keyword to filter on aggregate
properties.</li>
<li>Use a <code>VIEW</code> to access the result of a query as though it
was a new table.</li>
</ul></div>
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