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href="/docs/0.16.1-incubating/comparisons/druid-vs-key-value.html">Apache Druid vs. Key/Value Stores (HBase/Cassandra/OpenTSDB)</a></li><li class="navListItem"><a class="navItem" href="/docs/0.16.1-incubating/comparisons/druid-vs-kudu.html">Apache Druid vs Kudu</a></li><li class="navListItem"><a class="navItem" href="/docs/0.16.1-incubating/comparisons/druid-vs-redshift.html">Apache Druid vs Redshift</a></li><li class="navListItem"><a class="navItem" href="/docs/0.16.1-incubating/comparisons/druid-vs-spark.html">Apache Druid vs Spark</a></li><li class="navListItem"><a class="navItem" href="/docs/0.16.1-incubating/comparisons/druid-vs-sql-on-hadoop.html">Apache Druid vs SQL-on-Hadoop</a></li><li class="navListItem"><a class="navItem" href="/docs/0.16.1-incubating/design/auth.html">Authentication and Authorization</a></li><li class="navListItem"><a class="navItem" href="/docs/0.16.1-incubating/design/broker.html">Broker</a></li><li class="navListItem"><a class="navItem" 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<p>Virtual columns are queryable column &quot;views&quot; created from a set of columns during a query.</p>
<p>A virtual column can potentially draw from multiple underlying columns, although a virtual column always presents itself as a single column.</p>
<p>Virtual columns can be used as dimensions or as inputs to aggregators.</p>
<p>Each Apache Druid (incubating) query can accept a list of virtual columns as a parameter. The following scan query is provided as an example:</p>
<pre><code class="hljs">{
 <span class="hljs-attr">"queryType"</span>: <span class="hljs-string">"scan"</span>,
 <span class="hljs-attr">"dataSource"</span>: <span class="hljs-string">"page_data"</span>,
 <span class="hljs-attr">"columns"</span>:[],
 <span class="hljs-attr">"virtualColumns"</span>: [
    {
      <span class="hljs-attr">"type"</span>: <span class="hljs-string">"expression"</span>,
      <span class="hljs-attr">"name"</span>: <span class="hljs-string">"fooPage"</span>,
      <span class="hljs-attr">"expression"</span>: <span class="hljs-string">"concat('foo' + page)"</span>,
      <span class="hljs-attr">"outputType"</span>: <span class="hljs-string">"STRING"</span>
    },
    {
      <span class="hljs-attr">"type"</span>: <span class="hljs-string">"expression"</span>,
      <span class="hljs-attr">"name"</span>: <span class="hljs-string">"tripleWordCount"</span>,
      <span class="hljs-attr">"expression"</span>: <span class="hljs-string">"wordCount * 3"</span>,
      <span class="hljs-attr">"outputType"</span>: <span class="hljs-string">"LONG"</span>
    }
  ],
 <span class="hljs-attr">"intervals"</span>: [
   <span class="hljs-string">"2013-01-01/2019-01-02"</span>
 ]
}
</code></pre>
<h2><a class="anchor" aria-hidden="true" id="virtual-column-types"></a><a href="#virtual-column-types" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Virtual column types</h2>
<h3><a class="anchor" aria-hidden="true" id="expression-virtual-column"></a><a href="#expression-virtual-column" aria-hidden="true" class="hash-link"><svg class="hash-link-icon" aria-hidden="true" height="16" version="1.1" viewBox="0 0 16 16" width="16"><path fill-rule="evenodd" d="M4 9h1v1H4c-1.5 0-3-1.69-3-3.5S2.55 3 4 3h4c1.45 0 3 1.69 3 3.5 0 1.41-.91 2.72-2 3.25V8.59c.58-.45 1-1.27 1-2.09C10 5.22 8.98 4 8 4H4c-.98 0-2 1.22-2 2.5S3 9 4 9zm9-3h-1v1h1c1 0 2 1.22 2 2.5S13.98 12 13 12H9c-.98 0-2-1.22-2-2.5 0-.83.42-1.64 1-2.09V6.25c-1.09.53-2 1.84-2 3.25C6 11.31 7.55 13 9 13h4c1.45 0 3-1.69 3-3.5S14.5 6 13 6z"></path></svg></a>Expression virtual column</h3>
<p>The expression virtual column has the following syntax:</p>
<pre><code class="hljs">{
  <span class="hljs-string">"type"</span>: <span class="hljs-string">"expression"</span>,
  <span class="hljs-string">"name"</span>: &lt;name of the virtual column&gt;,
  <span class="hljs-string">"expression"</span>: &lt;row expression&gt;,
  <span class="hljs-string">"outputType"</span>: &lt;output value<span class="hljs-built_in"> type </span>of expression&gt;
}
</code></pre>
<table>
<thead>
<tr><th>property</th><th>description</th><th>required?</th></tr>
</thead>
<tbody>
<tr><td>name</td><td>The name of the virtual column.</td><td>yes</td></tr>
<tr><td>expression</td><td>An <a href="/docs/0.16.1-incubating/misc/math-expr.html">expression</a> that takes a row as input and outputs a value for the virtual column.</td><td>yes</td></tr>
<tr><td>outputType</td><td>The expression's output will be coerced to this type. Can be LONG, FLOAT, DOUBLE, or STRING.</td><td>no, default is FLOAT</td></tr>
</tbody>
</table>
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