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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="distinct">
<title>DISTINCT Operator</title>
<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"/>
The <codeph>DISTINCT</codeph> operator in a <codeph>SELECT</codeph> statement filters the
result set to remove duplicates.
<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>
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.
<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"/>
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.