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Data Management with SQL for Ecologists
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2. Accessing Data With Queries
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<h1>Accessing Data With Queries</h1>
<p> Last updated on 2023-04-21 |
<a href="https://github.com/datacarpentry/sql-ecology-lesson/edit/main/episodes/01-sql-basic-queries.md" class="external-link">Edit this page <i aria-hidden="true" data-feather="edit"></i></a></p>
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<h3 class="card-title">Questions</h3>
<ul><li>How do I write a basic query in SQL?</li>
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<h3 class="card-title">Objectives</h3>
<ul><li>Write and build queries.</li>
<li>Filter data given various criteria.</li>
<li>Sort the results of a query.</li>
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<section id="writing-my-first-query"><h2 class="section-heading">Writing my first query<a class="anchor" aria-label="anchor" href="#writing-my-first-query"></a>
</h2>
<hr class="half-width"><p>Let’s start by using the <strong>surveys</strong> table. Here we have
data on every individual that was captured at the site, including when
they were captured, what plot they were captured on, their species ID,
sex and weight in grams.</p>
<p>Let’s write an SQL query that selects all of the columns in the
surveys table. SQL queries can be written in the box located under the
“Execute SQL” tab. Click on the right arrow above the query box to
execute the query. (You can also use the keyboard shortcut “Cmd-Enter”
on a Mac or “Ctrl-Enter” on a Windows machine to execute a query.) The
results are displayed in the box below your query. If you want to
display all of the columns in a table, use the wildcard *.</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="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 have capitalized the words SELECT and FROM because they are SQL
keywords. SQL is case insensitive, but it helps for readability, and is
good style.</p>
<p>If we want to select a single column, we can type the column name
instead of the wildcard *.</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="dt">year</span></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>If we want more information, we can add more columns to the list of
fields, right after SELECT:</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="dt">year</span>, <span class="dt">month</span>, <span class="dt">day</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>
<div class="section level3">
<h3 id="limiting-results">Limiting results<a class="anchor" aria-label="anchor" href="#limiting-results"></a></h3>
<p>Sometimes you don’t want to see all the results, you just want to get
a sense of what’s being returned. In that case, you can use a
<code>LIMIT</code> clause. In particular, you would want to do this if
you were working with large databases.</p>
<div class="codewrapper sourceCode" id="cb4">
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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="kw">SELECT</span> <span class="op">*</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a><span class="kw">LIMIT</span> <span class="dv">10</span>;</span></code></pre>
</div>
</div>
<div class="section level3">
<h3 id="unique-values">Unique values<a class="anchor" aria-label="anchor" href="#unique-values"></a></h3>
<p>If we want only the unique values so that we can quickly see what
species have been sampled we use <code>DISTINCT</code></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> <span class="kw">DISTINCT</span> species_id</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>If we select more than one column, then the distinct pairs of values
are returned</p>
<div class="codewrapper sourceCode" id="cb6">
<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="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="kw">DISTINCT</span> <span class="dt">year</span>, species_id</span>
<span id="cb6-2"><a href="#cb6-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
</div>
<div class="section level3">
<h3 id="calculated-values">Calculated values<a class="anchor" aria-label="anchor" href="#calculated-values"></a></h3>
<p>We can also do calculations with the values in a query. For example,
if we wanted to look at the mass of each individual on different dates,
but we needed it in kg instead of g we would use</p>
<div class="codewrapper sourceCode" id="cb7">
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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>, <span class="dt">month</span>, <span class="dt">day</span>, weight<span class="op">/</span><span class="dv">1000</span></span>
<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
<p>When we run the query, the expression <code>weight / 1000</code> is
evaluated for each row and appended to that row, in a new column. If we
used the <code>INTEGER</code> data type for the weight field then
integer division would have been done, to obtain the correct results in
that case divide by <code>1000.0</code>. Expressions can use any fields,
any arithmetic operators (<code>+</code>, <code>-</code>,
<code>*</code>, and <code>/</code>) and a variety of built-in functions.
For example, we could round the values to make them easier to read.</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> plot_id, species_id, sex, weight, <span class="fu">ROUND</span>(weight <span class="op">/</span> <span class="dv">1000</span>, <span class="dv">2</span>)</span>
<span id="cb8-2"><a href="#cb8-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys;</span></code></pre>
</div>
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<h3 class="callout-title">Challenge<a class="anchor" aria-label="anchor" href="#challenge"></a>
</h3>
<div class="callout-content">
<ul><li>Write a query that returns the year, month, day, species_id and
weight in mg.</li>
</ul></div>
</div>
</div>
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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="dt">day</span>, <span class="dt">month</span>, <span class="dt">year</span>, species_id, weight <span class="op">*</span> <span class="dv">1000</span></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>
</div>
</div>
</div>
</div>
</div>
</section><section id="filtering"><h2 class="section-heading">Filtering<a class="anchor" aria-label="anchor" href="#filtering"></a>
</h2>
<hr class="half-width"><p>Databases can also filter data – selecting only the data meeting
certain criteria. For example, let’s say we only want data for the
species <em>Dipodomys merriami</em>, which has a species code of DM. We
need to add a <code>WHERE</code> clause to our query:</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="op">*</span></span>
<span id="cb10-2"><a href="#cb10-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb10-3"><a href="#cb10-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> species_id<span class="op">=</span><span class="st">'DM'</span>;</span></code></pre>
</div>
<p>We can do the same thing with numbers. Here, we only want the data
since 2000:</p>
<div class="codewrapper sourceCode" id="cb11">
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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 class="kw">FROM</span> surveys</span>
<span id="cb11-2"><a href="#cb11-2" 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></code></pre>
</div>
<p>If we used the <code>TEXT</code> data type for the year, the
<code>WHERE</code> clause should be <code>year >= '2000'</code>.</p>
<p>We can use more sophisticated conditions by combining tests with
<code>AND</code> and <code>OR</code>. For example, suppose we want the
data on <em>Dipodomys merriami</em> starting in the year 2000:</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</span>
<span id="cb12-3"><a href="#cb12-3" 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> (species_id <span class="op">=</span> <span class="st">'DM'</span>);</span></code></pre>
</div>
<p>Note that the parentheses are not needed, but again, they help with
readability. They also ensure that the computer combines
<code>AND</code> and <code>OR</code> in the way that we intend.</p>
<p>If we wanted to get data for any of the <em>Dipodomys</em> species,
which have species codes <code>DM</code>, <code>DO</code>, and
<code>DS</code>, we could combine the tests using OR:</p>
<div class="codewrapper sourceCode" id="cb13">
<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="cb13-1"><a href="#cb13-1" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span></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">WHERE</span> (species_id <span class="op">=</span> <span class="st">'DM'</span>) <span class="kw">OR</span> (species_id <span class="op">=</span> <span class="st">'DO'</span>) <span class="kw">OR</span> (species_id <span class="op">=</span> <span class="st">'DS'</span>);</span></code></pre>
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</h3>
<div class="callout-content">
<ul><li>Produce a table listing the data for all individuals in Plot 1 that
weighed more than 75 grams, telling us the date, species id code, and
weight (in kg).</li>
</ul></div>
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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> <span class="dt">day</span>, <span class="dt">month</span>, <span class="dt">year</span>, species_id, weight <span class="op">/</span> <span class="dv">1000</span></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">WHERE</span> (plot_id <span class="op">=</span> <span class="dv">1</span>) <span class="kw">AND</span> (weight <span class="op">></span> <span class="dv">75</span>);</span></code></pre>
</div>
</div>
</div>
</div>
</div>
</section><section id="building-more-complex-queries"><h2 class="section-heading">Building more complex queries<a class="anchor" aria-label="anchor" href="#building-more-complex-queries"></a>
</h2>
<hr class="half-width"><p>Now, let’s combine the above queries to get data for the 3
<em>Dipodomys</em> species from the year 2000 on. This time, let’s use
IN as one way to make the query easier to understand. It is equivalent
to saying
<code>WHERE (species_id = 'DM') OR (species_id = 'DO') OR (species_id = 'DS')</code>,
but reads more neatly:</p>
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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> <span class="op">*</span></span>
<span id="cb15-2"><a href="#cb15-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb15-3"><a href="#cb15-3" 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> (species_id <span class="kw">IN</span> (<span class="st">'DM'</span>, <span class="st">'DO'</span>, <span class="st">'DS'</span>));</span></code></pre>
</div>
<p>We started with something simple, then added more clauses one by one,
testing their effects as we went along. For complex queries, this is a
good strategy, to make sure you are getting what you want. Sometimes it
might help to take a subset of the data that you can easily see in a
temporary database to practice your queries on before working on a
larger or more complicated database.</p>
<p>When the queries become more complex, it can be useful to add
comments. In SQL, comments are started by <code>--</code>, and end at
the end of the line. For example, a commented version of the above query
can be written as:</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="co">-- Get post 2000 data on Dipodomys' species</span></span>
<span id="cb16-2"><a href="#cb16-2" aria-hidden="true" tabindex="-1"></a><span class="co">-- These are in the surveys table, and we are interested in all columns</span></span>
<span id="cb16-3"><a href="#cb16-3" aria-hidden="true" tabindex="-1"></a><span class="kw">SELECT</span> <span class="op">*</span> <span class="kw">FROM</span> surveys</span>
<span id="cb16-4"><a href="#cb16-4" aria-hidden="true" tabindex="-1"></a><span class="co">-- Sampling year is in the column `year`, and we want to include 2000</span></span>
<span id="cb16-5"><a href="#cb16-5" 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>
<span id="cb16-6"><a href="#cb16-6" aria-hidden="true" tabindex="-1"></a><span class="co">-- Dipodomys' species have the `species_id` DM, DO, and DS</span></span>
<span id="cb16-7"><a href="#cb16-7" aria-hidden="true" tabindex="-1"></a><span class="kw">AND</span> (species_id <span class="kw">IN</span> (<span class="st">'DM'</span>, <span class="st">'DO'</span>, <span class="st">'DS'</span>));</span></code></pre>
</div>
<p>Although SQL queries often read like plain English, it is
<em>always</em> useful to add comments; this is especially true of more
complex queries.</p>
</section><section id="sorting"><h2 class="section-heading">Sorting<a class="anchor" aria-label="anchor" href="#sorting"></a>
</h2>
<hr class="half-width"><p>We can also sort the results of our queries by using
<code>ORDER BY</code>. For simplicity, let’s go back to the
<strong>species</strong> table and alphabetize it by taxa.</p>
<p>First, let’s look at what’s in the <strong>species</strong> table.
It’s a table of the species_id and the full genus, species and taxa
information for each species_id. Having this in a separate table is
nice, because we didn’t need to include all this information in our main
<strong>surveys</strong> table.</p>
<div class="codewrapper sourceCode" id="cb17">
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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">SELECT</span> <span class="op">*</span></span>
<span id="cb17-2"><a href="#cb17-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> species;</span></code></pre>
</div>
<p>Now let’s order it by taxa.</p>
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<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> species</span>
<span id="cb18-3"><a href="#cb18-3" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> taxa <span class="kw">ASC</span>;</span></code></pre>
</div>
<p>The keyword <code>ASC</code> tells us to order it in ascending order.
We could alternately use <code>DESC</code> to get descending order.</p>
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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="op">*</span></span>
<span id="cb19-2"><a href="#cb19-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> species</span>
<span id="cb19-3"><a href="#cb19-3" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> taxa <span class="kw">DESC</span>;</span></code></pre>
</div>
<p><code>ASC</code> is the default.</p>
<p>We can also sort on several fields at once. To truly be alphabetical,
we might want to order by genus then species.</p>
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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="op">*</span></span>
<span id="cb20-2"><a href="#cb20-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> species</span>
<span id="cb20-3"><a href="#cb20-3" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> genus <span class="kw">ASC</span>, species <span class="kw">ASC</span>;</span></code></pre>
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</h3>
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<ul><li>Write a query that returns year, species_id, and weight in kg from
the surveys table, sorted with the largest weights at the top.</li>
</ul></div>
</div>
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<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="dt">year</span>, species_id, weight <span class="op">/</span> <span class="dv">1000</span></span>
<span id="cb21-2"><a href="#cb21-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb21-3"><a href="#cb21-3" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> weight <span class="kw">DESC</span>;</span></code></pre>
</div>
</div>
</div>
</div>
</div>
</section><section id="order-of-execution"><h2 class="section-heading">Order of execution<a class="anchor" aria-label="anchor" href="#order-of-execution"></a>
</h2>
<hr class="half-width"><p>Another note for ordering. We don’t actually have to display a column
to sort by it. For example, let’s say we want to order the birds by
their species ID, but we only want to see genus and species.</p>
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<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> genus, species</span>
<span id="cb22-2"><a href="#cb22-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> species</span>
<span id="cb22-3"><a href="#cb22-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> taxa <span class="op">=</span> <span class="st">'Bird'</span></span>
<span id="cb22-4"><a href="#cb22-4" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> species_id <span class="kw">ASC</span>;</span></code></pre>
</div>
<p>We can do this because sorting occurs earlier in the computational
pipeline than field selection.</p>
<p>The computer is basically doing this:</p>
<ol style="list-style-type: decimal"><li>Filtering rows according to WHERE</li>
<li>Sorting results according to ORDER BY</li>
<li>Displaying requested columns or expressions.</li>
</ol><p>Clauses are written in a fixed order: <code>SELECT</code>,
<code>FROM</code>, <code>WHERE</code>, then <code>ORDER BY</code>.</p>
<div id="multiple-statements" class="callout discussion">
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<h3 class="callout-title">Multiple statements<a class="anchor" aria-label="anchor" href="#multiple-statements"></a>
</h3>
<div class="callout-content">
<p>It is possible to write a query as a single line, but for
readability, we recommend to put each clause on its own line. The
standard way to separate a whole SQL statement is with a semicolon. This
allows more than one SQL statement to be executed together.</p>
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<ul><li>Let’s try to combine what we’ve learned so far in a single query.
Using the surveys table, write a query to display the three date fields,
<code>species_id</code>, and weight in kilograms (rounded to two decimal
places), for individuals captured in 1999, ordered alphabetically by the
<code>species_id</code>.</li>
<li>Write the query as a single line, then put each clause on its own
line, and see how more legible the query becomes!</li>
</ul></div>
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Show me the solution
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<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="dt">year</span>, <span class="dt">month</span>, <span class="dt">day</span>, species_id, <span class="fu">ROUND</span>(weight <span class="op">/</span> <span class="dv">1000</span>, <span class="dv">2</span>)</span>
<span id="cb23-2"><a href="#cb23-2" aria-hidden="true" tabindex="-1"></a><span class="kw">FROM</span> surveys</span>
<span id="cb23-3"><a href="#cb23-3" aria-hidden="true" tabindex="-1"></a><span class="kw">WHERE</span> <span class="dt">year</span> <span class="op">=</span> <span class="dv">1999</span></span>
<span id="cb23-4"><a href="#cb23-4" aria-hidden="true" tabindex="-1"></a><span class="kw">ORDER</span> <span class="kw">BY</span> species_id;</span></code></pre>
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<ul><li>It is useful to apply conventions when writing SQL queries to aid
readability.</li>
<li>Use logical connectors such as AND or OR to create more complex
queries.</li>
<li>Calculations using mathematical symbols can also be performed on SQL
queries.</li>
<li>Adding comments in SQL helps keep complex queries
understandable.</li>
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