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d484d2f684
This now gives a clean RAT check with bin/check-rat-report.py, which is one way for the Impala community to check compliance with ASF rules on intellectual property. Change-Id: I2ad06435f84a65ba126759e42a18fdaf52cd7036 Reviewed-on: http://gerrit.cloudera.org:8080/5232 Reviewed-by: Jim Apple <jbapple-impala@apache.org> Tested-by: Impala Public Jenkins Reviewed-by: John Russell <jrussell@cloudera.com>
80 lines
3.2 KiB
XML
80 lines
3.2 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="distinct">
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<title>DISTINCT Operator</title>
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<prolog>
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<metadata>
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<data name="Category" value="Impala"/>
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<data name="Category" value="SQL"/>
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<data name="Category" value="Querying"/>
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<data name="Category" value="Aggregate Functions"/>
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<data name="Category" value="Developers"/>
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<data name="Category" value="Data Analysts"/>
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</metadata>
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</prolog>
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<conbody>
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<p>
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<indexterm audience="Cloudera">DISTINCT operator</indexterm>
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The <codeph>DISTINCT</codeph> operator in a <codeph>SELECT</codeph> statement filters the result set to
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remove duplicates:
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</p>
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<codeblock>-- Returns the unique values from one column.
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-- NULL is included in the set of values if any rows have a NULL in this column.
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select distinct c_birth_country from customer;
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-- Returns the unique combinations of values from multiple columns.
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select distinct c_salutation, c_last_name from customer;</codeblock>
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<p>
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You can use <codeph>DISTINCT</codeph> in combination with an aggregation function, typically
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<codeph>COUNT()</codeph>, to find how many different values a column contains:
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</p>
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<codeblock>-- Counts the unique values from one column.
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-- NULL is not included as a distinct value in the count.
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select count(distinct c_birth_country) from customer;
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-- Counts the unique combinations of values from multiple columns.
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select count(distinct c_salutation, c_last_name) from customer;</codeblock>
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<p>
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One construct that Impala SQL does <i>not</i> support is using <codeph>DISTINCT</codeph> in more than one
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aggregation function in the same query. For example, you could not have a single query with both
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<codeph>COUNT(DISTINCT c_first_name)</codeph> and <codeph>COUNT(DISTINCT c_last_name)</codeph> in the
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<codeph>SELECT</codeph> list.
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</p>
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<p conref="../shared/impala_common.xml#common/zero_length_strings"/>
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<note conref="../shared/impala_common.xml#common/multiple_count_distinct"/>
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<note>
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<p>
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In contrast with some database systems that always return <codeph>DISTINCT</codeph> values in sorted order,
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Impala does not do any ordering of <codeph>DISTINCT</codeph> values. Always include an <codeph>ORDER
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BY</codeph> clause if you need the values in alphabetical or numeric sorted order.
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</p>
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</note>
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</conbody>
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</concept>
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