docs/topics/impala_distinct.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="distinct">

  <title>DISTINCT Operator</title>

  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Aggregate Functions"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      The <codeph>DISTINCT</codeph> operator in a <codeph>SELECT</codeph> statement filters the
      result set to remove duplicates.
    </p>

<codeblock>-- Returns the unique values from one column.
-- NULL is included in the set of values if any rows have a NULL in this column.
SELECT DISTINCT c_birth_country FROM customer;

-- Returns the unique combinations of values from multiple columns.
SELECT DISTINCT c_salutation, c_last_name FROM customer;</codeblock>

    <p>
      You can use <codeph>DISTINCT</codeph> in combination with an aggregation function,
      typically <codeph>COUNT()</codeph>, to find how many different values a column contains.
    </p>

<codeblock>-- Counts the unique values from one column.
-- NULL is not included as a distinct value in the count.
SELECT COUNT(DISTINCT c_birth_country) FROM customer;

-- Counts the unique combinations of values from multiple columns.
SELECT COUNT(DISTINCT c_salutation, c_last_name) FROM customer;</codeblock>

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

    <note>
      <p>
        In contrast with some database systems that always return <codeph>DISTINCT</codeph>
        values in sorted order, Impala does not do any ordering of <codeph>DISTINCT</codeph>
        values. Always include an <codeph>ORDER BY</codeph> clause if you need the values in
        alphabetical or numeric sorted order.
      </p>
    </note>

  </conbody>

</concept>