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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>
85 lines
3.3 KiB
XML
85 lines
3.3 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 rev="1.1" id="with">
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<title>WITH Clause</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="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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A clause that can be added before a <codeph>SELECT</codeph> statement, to define aliases for complicated
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expressions that are referenced multiple times within the body of the <codeph>SELECT</codeph>. Similar to
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<codeph>CREATE VIEW</codeph>, except that the table and column names defined in the <codeph>WITH</codeph>
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clause do not persist after the query finishes, and do not conflict with names used in actual tables or
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views. Also known as <q>subquery factoring</q>.
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</p>
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<p>
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You can rewrite a query using subqueries to work the same as with the <codeph>WITH</codeph> clause. The
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purposes of the <codeph>WITH</codeph> clause are:
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</p>
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<ul>
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<li>
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Convenience and ease of maintenance from less repetition with the body of the query. Typically used with
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queries involving <codeph>UNION</codeph>, joins, or aggregation functions where the similar complicated
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expressions are referenced multiple times.
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</li>
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<li>
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SQL code that is easier to read and understand by abstracting the most complex part of the query into a
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separate block.
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</li>
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<li>
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Improved compatibility with SQL from other database systems that support the same clause (primarily Oracle
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Database).
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<note>
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<p>
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The Impala <codeph>WITH</codeph> clause does not support recursive queries in the
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<codeph>WITH</codeph>, which is supported in some other database systems.
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</p>
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</note>
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</li>
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</ul>
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<p conref="../shared/impala_common.xml#common/sql1999"/>
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<p conref="../shared/impala_common.xml#common/example_blurb"/>
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<codeblock>-- Define 2 subqueries that can be referenced from the body of a longer query.
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with t1 as (select 1), t2 as (select 2) insert into tab select * from t1 union all select * from t2;
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-- Define one subquery at the outer level, and another at the inner level as part of the
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-- initial stage of the UNION ALL query.
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with t1 as (select 1) (with t2 as (select 2) select * from t2) union all select * from t1;</codeblock>
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</conbody>
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</concept>
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