docs/topics/impala_stddev.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="1.4" id="stddev">

  <title>STDDEV, STDDEV_SAMP, STDDEV_POP Functions</title>
  <titlealts audience="PDF"><navtitle>STDDEV, STDDEV_SAMP, STDDEV_POP</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Impala Functions"/>
      <data name="Category" value="Aggregate Functions"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">stddev() function</indexterm>
      <indexterm audience="hidden">stddev_samp() function</indexterm>
      <indexterm audience="hidden">stddev_pop() function</indexterm>
      An aggregate function that
      <xref href="http://en.wikipedia.org/wiki/Standard_deviation" scope="external" format="html">standard
      deviation</xref> of a set of numbers.
    </p>

    <p conref="../shared/impala_common.xml#common/syntax_blurb"/>

<codeblock>{ STDDEV | STDDEV_SAMP | STDDEV_POP } ([DISTINCT | ALL] <varname>expression</varname>)</codeblock>

    <p>
      This function works with any numeric data type.
    </p>

    <p conref="../shared/impala_common.xml#common/former_odd_return_type_string"/>

    <p>
      This function is typically used in mathematical formulas related to probability distributions.
    </p>

    <p>
      The <codeph>STDDEV_POP()</codeph> and <codeph>STDDEV_SAMP()</codeph> functions compute the population
      standard deviation and sample standard deviation, respectively, of the input values.
      (<codeph>STDDEV()</codeph> is an alias for <codeph>STDDEV_SAMP()</codeph>.) Both functions evaluate all input
      rows matched by the query. The difference is that <codeph>STDDEV_SAMP()</codeph> is scaled by
      <codeph>1/(N-1)</codeph> while <codeph>STDDEV_POP()</codeph> is scaled by <codeph>1/N</codeph>.
    </p>

    <p>
      If no input rows match the query, the result of any of these functions is <codeph>NULL</codeph>. If a single
      input row matches the query, the result of any of these functions is <codeph>"0.0"</codeph>.
    </p>

    <p conref="../shared/impala_common.xml#common/example_blurb"/>

    <p>
      This example demonstrates how <codeph>STDDEV()</codeph> and <codeph>STDDEV_SAMP()</codeph> return the same
      result, while <codeph>STDDEV_POP()</codeph> uses a slightly different calculation to reflect that the input
      data is considered part of a larger <q>population</q>.
    </p>

<codeblock>[localhost:21000] &gt; select stddev(score) from test_scores;
+---------------+
| stddev(score) |
+---------------+
| 28.5          |
+---------------+
[localhost:21000] &gt; select stddev_samp(score) from test_scores;
+--------------------+
| stddev_samp(score) |
+--------------------+
| 28.5               |
+--------------------+
[localhost:21000] &gt; select stddev_pop(score) from test_scores;
+-------------------+
| stddev_pop(score) |
+-------------------+
| 28.4858           |
+-------------------+
</codeblock>

    <p>
      This example demonstrates that, because the return value of these aggregate functions is a
      <codeph>STRING</codeph>, you must currently convert the result with <codeph>CAST</codeph>.
    </p>

<codeblock>[localhost:21000] &gt; create table score_stats as select cast(stddev(score) as decimal(7,4)) `standard_deviation`, cast(variance(score) as decimal(7,4)) `variance` from test_scores;
+-------------------+
| summary           |
+-------------------+
| Inserted 1 row(s) |
+-------------------+
[localhost:21000] &gt; desc score_stats;
+--------------------+--------------+---------+
| name               | type         | comment |
+--------------------+--------------+---------+
| standard_deviation | decimal(7,4) |         |
| variance           | decimal(7,4) |         |
+--------------------+--------------+---------+
</codeblock>

    <p conref="../shared/impala_common.xml#common/restrictions_blurb"/>

    <p conref="../shared/impala_common.xml#common/analytic_not_allowed_caveat"/>

    <p conref="../shared/impala_common.xml#common/related_info"/>

    <p>
      The <codeph>STDDEV()</codeph>, <codeph>STDDEV_POP()</codeph>, and <codeph>STDDEV_SAMP()</codeph> functions
      compute the standard deviation (square root of the variance) based on the results of
      <codeph>VARIANCE()</codeph>, <codeph>VARIANCE_POP()</codeph>, and <codeph>VARIANCE_SAMP()</codeph>
      respectively. See <xref href="impala_variance.xml#variance"/> for details about the variance property.
    </p>
  </conbody>
</concept>