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Data Management with SQL for Ecologists
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Data Management with SQL for Ecologists
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<section id="aio-00-sql-introduction"><p>Content from <a href="00-sql-introduction.html">Introducing Databases and SQL</a></p>
<hr>
<p> Last updated on 2023-04-21 |
<a href="https://github.com/datacarpentry/sql-ecology-lesson/edit/main/episodes/00-sql-introduction.md" class="external-link">Edit this page <i aria-hidden="true" data-feather="edit"></i></a></p>
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<li>What is a relational database and why should I use it?</li>
<li>What is SQL?</li>
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<li>Describe why relational databases are useful.</li>
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<li>Define SQLite data types.</li>
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<div class="section level3">
<h3 id="setup">Setup<a class="anchor" aria-label="anchor" href="#setup"></a>
</h3>
<p><em>Note: this should have been done by participants before the start
of the workshop.</em></p>
<p>We use <a href="https://sqlitebrowser.org/" class="external-link">DB Browser for SQLite</a>
and the <a href="https://figshare.com/articles/Portal_Project_Teaching_Database/1314459" class="external-link">Portal
Project dataset</a> throughout this lesson. See <a href="index.html#setup">Setup</a> for instructions on how to download the
data, and also how to install DB Browser for SQLite.</p>
</div>
<section id="motivation"><h2 class="section-heading">Motivation<a class="anchor" aria-label="anchor" href="#motivation"></a>
</h2>
<hr class="half-width">
<p>To start, let’s orient ourselves in our project workflow. Previously,
we used Excel and OpenRefine to go from messy, human created data to
cleaned, computer-readable data. Now we’re going to move to the next
piece of the data workflow, using the computer to read in our data, and
then use it for analysis and visualization.</p>
<div class="section level3">
<h3 id="what-is-sql">What is SQL?<a class="anchor" aria-label="anchor" href="#what-is-sql"></a>
</h3>
<p>SQL stands for Structured Query Language. SQL allows us to interact
with relational databases through queries. These queries can allow you
to perform a number of actions such as: insert, select, update and
delete information in a database.</p>
</div>
<div class="section level3">
<h3 id="dataset-description">Dataset Description<a class="anchor" aria-label="anchor" href="#dataset-description"></a>
</h3>
<p>The data we will be using is a time-series for a small mammal
community in southern Arizona. This is part of a project studying the
effects of rodents and ants on the plant community that has been running
for almost 40 years. The rodents are sampled on a series of 24 plots,
with different experimental manipulations controlling which rodents are
allowed to access which plots.</p>
<p>This is a real dataset that has been used in over 100 publications.
We’ve simplified it for the workshop, but you can download the <a href="https://esapubs.org/archive/ecol/E090/118/" class="external-link">full dataset</a> and
work with it using exactly the same tools we’ll learn about today.</p>
</div>
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<h3 id="questions">Questions<a class="anchor" aria-label="anchor" href="#questions"></a>
</h3>
<p>Let’s look at some of the cleaned spreadsheets you downloaded during
<a href="index.html#setup">Setup</a> to complete this challenge. You’ll need
the following three files:</p>
<ul>
<li><code>surveys.csv</code></li>
<li><code>species.csv</code></li>
<li><code>plots.csv</code></li>
</ul>
<div id="challenge" class="callout discussion">
<div class="callout-square">
<i class="callout-icon" data-feather="message-circle"></i>
</div>
<div id="challenge" class="callout-inner">
<h3 class="callout-title">Challenge<a class="anchor" aria-label="anchor" href="#challenge"></a>
</h3>
<div class="callout-content">
<p>Open each of these csv files and explore them. What information is
contained in each file? Specifically, if I had the following research
questions:</p>
<ul>
<li>How has the hindfoot length and weight of <em>Dipodomys</em> species
changed over time?</li>
<li>What is the average weight of each species, per year?</li>
<li>What information can I learn about <em>Dipodomys</em> species in the
2000s, over time?</li>
</ul>
<p>What would I need to answer these questions? Which files have the
data I need? What operations would I need to perform if I were doing
these analyses by hand?</p>
</div>
</div>
</div>
</div>
<div class="section level3">
<h3 id="goals">Goals<a class="anchor" aria-label="anchor" href="#goals"></a>
</h3>
<p>In order to answer the questions described above, we’ll need to do
the following basic data operations:</p>
<ul>
<li>select subsets of the data (rows and columns)</li>
<li>group subsets of data</li>
<li>do math and other calculations</li>
<li>combine data across spreadsheets</li>
</ul>
<p>In addition, we don’t want to do this manually! Instead of searching
for the right pieces of data ourselves, or clicking between
spreadsheets, or manually sorting columns, we want to make the computer
do the work.</p>
<p>In particular, we want to use a tool where it’s easy to repeat our
analysis in case our data changes. We also want to do all this searching
without actually modifying our source data.</p>
<p>Putting our data into a relational database and using SQL will help
us achieve these goals.</p>
<div id="definition-relational-database" class="callout callout">
<div class="callout-square">
<i class="callout-icon" data-feather="bell"></i>
</div>
<div id="definition-relational-database" class="callout-inner">
<h3 class="callout-title">Definition: <em>Relational Database</em><a class="anchor" aria-label="anchor" href="#definition-relational-database"></a>
</h3>
<div class="callout-content">
<p>A relational database stores data in <em>relations</em> made up of
<em>records</em> with <em>fields</em>. The relations are usually
represented as <em>tables</em>; each record is usually shown as a row,
and the fields as columns. In most cases, each record will have a unique
identifier, called a <em>key</em>, which is stored as one of its fields.
Records may also contain keys that refer to records in other tables,
which enables us to combine information from two or more sources.</p>
</div>
</div>
</div>
</div>
</section><section id="databases"><h2 class="section-heading">Databases<a class="anchor" aria-label="anchor" href="#databases"></a>
</h2>
<hr class="half-width">
<div class="section level3">
<h3 id="why-use-relational-databases">Why use relational databases<a class="anchor" aria-label="anchor" href="#why-use-relational-databases"></a>
</h3>
<p>Using a relational database serves several purposes.</p>
<ul>
<li>It keeps your data separate from your analysis.
<ul>
<li>This means there’s no risk of accidentally changing data when you
analyze it.</li>
<li>If we get new data we can rerun the query.</li>
</ul>
</li>
<li>It’s fast, even for large amounts of data.</li>
<li>It improves quality control of data entry (type constraints and use
of forms in MS Access, Filemaker, Oracle Application Express etc.)</li>
<li>The concepts of relational database querying are core to
understanding how to do similar things using programming languages such
as R or Python.</li>
</ul>
</div>
<div class="section level3">
<h3 id="database-management-systems">Database Management Systems<a class="anchor" aria-label="anchor" href="#database-management-systems"></a>
</h3>
<p>There are different database management systems to work with
relational databases such as SQLite, MySQL, PostgreSQL, MSSQL Server,
and many more. Each of them differ mainly based on their scalability,
but they all share the same core principles of relational databases. In
this lesson, we use SQLite to introduce you to SQL and data retrieval
from a relational database.</p>
</div>
<div class="section level3">
<h3 id="relational-databases">Relational databases<a class="anchor" aria-label="anchor" href="#relational-databases"></a>
</h3>
<p>Let’s look at a pre-existing database, the
<code>portal_mammals.sqlite</code> file from the Portal Project dataset
that we downloaded during <a href="index.html#setup">Setup</a>. Click on the
“Open Database” button, select the portal_mammals.sqlite file, and click
“Open” to open the database.</p>
<p>You can see the tables in the database by looking at the left hand
side of the screen under Database Structure tab. Here you will see a
list under “Tables.” Each item listed here corresponds to one of the
<code>csv</code> files we were exploring earlier. To see the contents of
any table, right-click on it, and then click the “Browse Table” from the
menu, or select the “Browse Data” tab next to the “Database Structure”
tab and select the wanted table from the dropdown named “Table”. This
will give us a view that we’re used to - a copy of the table. Hopefully
this helps to show that a database is, in some sense, just a collection
of tables, where there’s some value in the tables that allows them to be
connected to each other (the “related” part of “relational
database”).</p>
<p>The “Database Structure” tab also provides some metadata about each
table. If you click on the down arrow next to a table name, you will see
information about the columns, which in databases are referred to as
“fields,” and their assigned data types. (The rows of a database table
are called <em>records</em>.) Each field contains one variety or type of
data, often numbers or text. You can see in the <code>surveys</code>
table that most fields contain numbers (BIGINT, or big integer, and
FLOAT, or floating point numbers/decimals) while the
<code>species</code> table is entirely made up of text fields.</p>
<p>The “Execute SQL” tab is blank now - this is where we’ll be typing
our queries to retrieve information from the database tables.</p>
<p>To summarize:</p>
<ul>
<li>Relational databases store data in tables with fields (columns) and
records (rows)</li>
<li>Data in tables has types, and all values in a field have the same
type (<a href="#datatypes">list of data types</a>)</li>
<li>Queries let us look up data or make calculations based on
columns</li>
</ul>
</div>
<div class="section level3">
<h3 id="database-design">Database Design<a class="anchor" aria-label="anchor" href="#database-design"></a>
</h3>
<ul>
<li>Every row-column combination contains a single <em>atomic</em>
value, i.e., not containing parts we might want to work with
separately.</li>
<li>One field per type of information</li>
<li>No redundant information
<ul>
<li>Split into separate tables with one table per class of
information</li>
<li>Needs an identifier in common between tables – shared column - to
reconnect (known as a <em>foreign key</em>).</li>
</ul>
</li>
</ul>
</div>
<div class="section level3">
<h3 id="import">Import<a class="anchor" aria-label="anchor" href="#import"></a>
</h3>
<p>Before we get started with writing our own queries, we’ll create our
own database. We’ll be creating this database from the three
<code>csv</code> files we downloaded earlier. Close the currently open
database (<strong>File > Close Database</strong>) and then follow
these instructions:</p>
<ol style="list-style-type: decimal">
<li>Start a New Database</li>
</ol>
<ul>
<li>Click the <strong>New Database</strong> button</li>
<li>Give a name and click Save to create the database in the opened
folder</li>
<li>In the “Edit table definition” window that pops up, click cancel as
we will be importing tables, not creating them from scratch</li>
</ul>
<ol start="2" style="list-style-type: decimal">
<li>Select <strong>File >> Import >> Table from CSV
file…</strong>
</li>
<li>Choose <code>surveys.csv</code> from the data folder we downloaded
and click <strong>Open</strong>.</li>
<li>Give the table a name that matches the file name
(<code>surveys</code>), or use the default</li>
<li>If the first row has column headings, be sure to check the box next
to “Column names in first line”.</li>
<li>Be sure the field separator and quotation options are correct. If
you’re not sure which options are correct, test some of the options
until the preview at the bottom of the window looks right.</li>
<li>Press <strong>OK</strong>, you should subsequently get a message
that the table was imported.</li>
<li>Back on the Database Structure tab, you should now see the table
listed. Right click on the table name and choose <strong>Modify
Table</strong>, or click on the <strong>Modify Table</strong> button
just under the tabs and above the table list.</li>
<li>Click <strong>Save</strong> if asked to save all pending
changes.</li>
<li>In the center panel of the window that appears, set the data types
for each field using the suggestions in the table below (this includes
fields from the <code>plots</code> and <code>species</code> tables
also).</li>
<li>Finally, click <strong>OK</strong> one more time to confirm the
operation. Then click the <strong>Write Changes</strong> button to save
the database.</li>
</ol>
<table style="width:100%;" class="table">
<colgroup>
<col width="21%">
<col width="42%">
<col width="29%">
<col width="6%">
</colgroup>
<thead><tr class="header">
<th>Field</th>
<th align="left">Data Type</th>
<th>Motivation</th>
<th>Table(s)</th>
</tr></thead>
<tbody>
<tr class="odd">
<td>day</td>
<td align="left">INTEGER</td>
<td>Having data as numeric allows for meaningful arithmetic and
comparisons</td>
<td>surveys</td>
</tr>
<tr class="even">
<td>genus</td>
<td align="left">TEXT</td>
<td>Field contains text data</td>
<td>species</td>
</tr>
<tr class="odd">
<td>hindfoot_length</td>
<td align="left">REAL</td>
<td>Field contains measured numeric data</td>
<td>surveys</td>
</tr>
<tr class="even">
<td>month</td>
<td align="left">INTEGER</td>
<td>Having data as numeric allows for meaningful arithmetic and
comparisons</td>
<td>surveys</td>
</tr>
<tr class="odd">
<td>plot_id</td>
<td align="left">INTEGER</td>
<td>Field contains numeric data</td>
<td>plots, surveys</td>
</tr>
<tr class="even">
<td>plot_type</td>
<td align="left">TEXT</td>
<td>Field contains text data</td>
<td>plots</td>
</tr>
<tr class="odd">
<td>record_id</td>
<td align="left">INTEGER</td>
<td>Field contains numeric data</td>
<td>surveys</td>
</tr>
<tr class="even">
<td>sex</td>
<td align="left">TEXT</td>
<td>Field contains text data</td>
<td>surveys</td>
</tr>
<tr class="odd">
<td>species_id</td>
<td align="left">TEXT</td>
<td>Field contains text data</td>
<td>species, surveys</td>
</tr>
<tr class="even">
<td>species</td>
<td align="left">TEXT</td>
<td>Field contains text data</td>
<td>species</td>
</tr>
<tr class="odd">
<td>taxa</td>
<td align="left">TEXT</td>
<td>Field contains text data</td>
<td>species</td>
</tr>
<tr class="even">
<td>weight</td>
<td align="left">REAL</td>
<td>Field contains measured numerical data</td>
<td>surveys</td>
</tr>
<tr class="odd">
<td>year</td>
<td align="left">INTEGER</td>
<td>Allows for meaningful arithmetic and comparisons</td>
<td>surveys</td>
</tr>
</tbody>
</table>
<div id="challenge-1" class="callout discussion">
<div class="callout-square">
<i class="callout-icon" data-feather="message-circle"></i>
</div>
<div id="challenge-1" class="callout-inner">
<h3 class="callout-title">Challenge<a class="anchor" aria-label="anchor" href="#challenge-1"></a>
</h3>
<div class="callout-content">
<ul>
<li>Import the <code>plots</code> and <code>species</code> tables</li>
</ul>
</div>
</div>
</div>
<p>You can also use this same approach to append new fields to an
existing table.</p>
</div>
<div class="section level3">
<h3 id="adding-fields-to-existing-tables">Adding fields to existing tables<a class="anchor" aria-label="anchor" href="#adding-fields-to-existing-tables"></a>
</h3>
<ol style="list-style-type: decimal">
<li>Go to the “Database Structure” tab, right click on the table you’d
like to add data to, and choose <strong>Modify Table</strong>, or click
on the <strong>Modify Table</strong> just under the tabs and above the
table.</li>
<li>Click the <strong>Add Field</strong> button to add a new field and
assign it a data type.</li>
</ol>
</div>
<div class="section level3">
<h3 id="datatypes">Data types<a class="anchor" aria-label="anchor" href="#datatypes"></a>
</h3>
<table class="table">
<colgroup>
<col width="33%">
<col width="66%">
</colgroup>
<thead><tr class="header">
<th>Data type</th>
<th align="left">Description</th>
</tr></thead>
<tbody>
<tr class="odd">
<td>CHARACTER(n)</td>
<td align="left">Character string. Fixed-length n</td>
</tr>
<tr class="even">
<td>VARCHAR(n) or CHARACTER VARYING(n)</td>
<td align="left">Character string. Variable length. Maximum length
n</td>
</tr>
<tr class="odd">
<td>BINARY(n)</td>
<td align="left">Binary string. Fixed-length n</td>
</tr>
<tr class="even">
<td>BOOLEAN</td>
<td align="left">Stores TRUE or FALSE values</td>
</tr>
<tr class="odd">
<td>VARBINARY(n) or BINARY VARYING(n)</td>
<td align="left">Binary string. Variable length. Maximum length n</td>
</tr>
<tr class="even">
<td>INTEGER(p)</td>
<td align="left">Integer numerical (no decimal).</td>
</tr>
<tr class="odd">
<td>SMALLINT</td>
<td align="left">Integer numerical (no decimal).</td>
</tr>
<tr class="even">
<td>INTEGER</td>
<td align="left">Integer numerical (no decimal).</td>
</tr>
<tr class="odd">
<td>BIGINT</td>
<td align="left">Integer numerical (no decimal).</td>
</tr>
<tr class="even">
<td>DECIMAL(p,s)</td>
<td align="left">Exact numerical, precision p, scale s.</td>
</tr>
<tr class="odd">
<td>NUMERIC(p,s)</td>
<td align="left">Exact numerical, precision p, scale s. (Same as
DECIMAL)</td>
</tr>
<tr class="even">
<td>FLOAT(p)</td>
<td align="left">Approximate numerical, mantissa precision p. A floating
number in base 10 exponential notation.</td>
</tr>
<tr class="odd">
<td>REAL</td>
<td align="left">Approximate numerical</td>
</tr>
<tr class="even">
<td>FLOAT</td>
<td align="left">Approximate numerical</td>
</tr>
<tr class="odd">
<td>DOUBLE PRECISION</td>
<td align="left">Approximate numerical</td>
</tr>
<tr class="even">
<td>DATE</td>
<td align="left">Stores year, month, and day values</td>
</tr>
<tr class="odd">
<td>TIME</td>
<td align="left">Stores hour, minute, and second values</td>
</tr>
<tr class="even">
<td>TIMESTAMP</td>
<td align="left">Stores year, month, day, hour, minute, and second
values</td>
</tr>
<tr class="odd">
<td>INTERVAL</td>
<td align="left">Composed of a number of integer fields, representing a
period of time, depending on the type of interval</td>
</tr>
<tr class="even">
<td>ARRAY</td>
<td align="left">A set-length and ordered collection of elements</td>
</tr>
<tr class="odd">
<td>MULTISET</td>
<td align="left">A variable-length and unordered collection of
elements</td>
</tr>
<tr class="even">
<td>XML</td>
<td align="left">Stores XML data</td>
</tr>
</tbody>
</table>
</div>
<div class="section level3">
<h3 id="datatypediffs">SQL Data Type Quick Reference<a class="anchor" aria-label="anchor" href="#datatypediffs"></a>
</h3>
<p>Different databases offer different choices for the data type
definition.</p>
<p>The following table shows some of the common names of data types
between the various database platforms:</p>
<table class="table">
<colgroup>
<col width="19%">
<col width="38%">
<col width="26%">
<col width="5%">
<col width="4%">
<col width="4%">
</colgroup>
<thead><tr class="header">
<th align="left">Data type</th>
<th align="left">Access</th>
<th align="left">SQLServer</th>
<th align="left">Oracle</th>
<th align="left">MySQL</th>
<th align="left">PostgreSQL</th>
</tr></thead>
<tbody>
<tr class="odd">
<td align="left">boolean</td>
<td align="left">Yes/No</td>
<td align="left">Bit</td>
<td align="left">Byte</td>
<td align="left">N/A</td>
<td align="left">Boolean</td>
</tr>
<tr class="even">
<td align="left">integer</td>
<td align="left">Number (integer)</td>
<td align="left">Int</td>
<td align="left">Number</td>
<td align="left">Int / Integer</td>
<td align="left">Int / Integer</td>
</tr>
<tr class="odd">
<td align="left">float</td>
<td align="left">Number (single)</td>
<td align="left">Float / Real</td>
<td align="left">Number</td>
<td align="left">Float</td>
<td align="left">Numeric</td>
</tr>
<tr class="even">
<td align="left">currency</td>
<td align="left">Currency</td>
<td align="left">Money</td>
<td align="left">N/A</td>
<td align="left">N/A</td>
<td align="left">Money</td>
</tr>
<tr class="odd">
<td align="left">string (fixed)</td>
<td align="left">N/A</td>
<td align="left">Char</td>
<td align="left">Char</td>
<td align="left">Char</td>
<td align="left">Char</td>
</tr>
<tr class="even">
<td align="left">string (variable)</td>
<td align="left">Text (<256) / Memo (65k+)</td>
<td align="left">Varchar</td>
<td align="left">Varchar2</td>
<td align="left">Varchar</td>
<td align="left">Varchar</td>
</tr>
<tr class="odd">
<td align="left">binary object OLE Object Memo Binary (fixed up to
8K)</td>
<td align="left">Varbinary (<8K)</td>
<td align="left">Image (<2GB) Long</td>
<td align="left">Raw Blob</td>
<td align="left">Text Binary</td>
<td align="left">Varbinary</td>
</tr>
</tbody>
</table>
<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>SQL allows us to select and group subsets of data, do math and other
calculations, and combine data.</li>
<li>A relational database is made up of tables which are related to each
other by shared keys.</li>
<li>Different database management systems (DBMS) use slightly different
vocabulary, but they are all based on the same ideas.</li>
</ul>
</div>
</div>
</div>
</div>
</section></section><section id="aio-01-sql-basic-queries"><p>Content from <a href="01-sql-basic-queries.html">Accessing Data With Queries</a></p>
<hr>
<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>
<div class="text-end">
<button role="button" aria-pressed="false" tabindex="0" id="expand-code" class="pull-right"> Expand All Solutions <i aria-hidden="true" data-feather="plus"></i></button>
</div>
<div class="overview card">
<h2 class="card-header">Overview</h2>
<div class="row g-0">
<div class="col-md-4">
<div class="card-body">
<div class="inner">
<h3 class="card-title">Questions</h3>
<ul>
<li>How do I write a basic query in SQL?</li>
</ul>
</div>
</div>
</div>
<div class="col-md-8">
<div class="card-body">
<div class="inner bordered">
<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>
</ul>
</div>
</div>
</div>
</div>
</div>
<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>
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<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="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>
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<h3 id="unique-values">Unique values<a class="anchor" aria-label="anchor" href="#unique-values"></a>
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<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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