impala/docs/topics/impala_create_view.xml

<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="1.1" id="create_view">

  <title>CREATE VIEW Statement</title>
  <titlealts audience="PDF"><navtitle>CREATE VIEW</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="DDL"/>
      <data name="Category" value="Tables"/>
      <data name="Category" value="Schemas"/>
      <data name="Category" value="Views"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">CREATE VIEW statement</indexterm>
      The <codeph>CREATE VIEW</codeph> statement lets you create a shorthand abbreviation for a more complicated
      query. The base query can involve joins, expressions, reordered columns, column aliases, and other SQL
      features that can make a query hard to understand or maintain.
    </p>

    <p>
      Because a view is purely a logical construct (an alias for a query) with no physical data behind it,
      <codeph>ALTER VIEW</codeph> only involves changes to metadata in the metastore database, not any data files
      in HDFS.
    </p>

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

<codeblock>CREATE VIEW [IF NOT EXISTS] <varname>view_name</varname> [(<varname>column_list</varname>)]
  AS <varname>select_statement</varname></codeblock>

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

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

    <p>
      The <codeph>CREATE VIEW</codeph> statement can be useful in scenarios such as the following:
    </p>

    <ul>
      <li>
        To turn even the most lengthy and complicated SQL query into a one-liner. You can issue simple queries
        against the view from applications, scripts, or interactive queries in <cmdname>impala-shell</cmdname>.
        For example:
<codeblock>select * from <varname>view_name</varname>;
select * from <varname>view_name</varname> order by c1 desc limit 10;</codeblock>
        The more complicated and hard-to-read the original query, the more benefit there is to simplifying the
        query using a view.
      </li>

      <li>
        To hide the underlying table and column names, to minimize maintenance problems if those names change. In
        that case, you re-create the view using the new names, and all queries that use the view rather than the
        underlying tables keep running with no changes.
      </li>

      <li>
        To experiment with optimization techniques and make the optimized queries available to all applications.
        For example, if you find a combination of <codeph>WHERE</codeph> conditions, join order, join hints, and so
        on that works the best for a class of queries, you can establish a view that incorporates the
        best-performing techniques. Applications can then make relatively simple queries against the view, without
        repeating the complicated and optimized logic over and over. If you later find a better way to optimize the
        original query, when you re-create the view, all the applications immediately take advantage of the
        optimized base query.
      </li>

      <li>
        To simplify a whole class of related queries, especially complicated queries involving joins between
        multiple tables, complicated expressions in the column list, and other SQL syntax that makes the query
        difficult to understand and debug. For example, you might create a view that joins several tables, filters
        using several <codeph>WHERE</codeph> conditions, and selects several columns from the result set.
        Applications might issue queries against this view that only vary in their <codeph>LIMIT</codeph>,
        <codeph>ORDER BY</codeph>, and similar simple clauses.
      </li>
    </ul>

    <p>
      For queries that require repeating complicated clauses over and over again, for example in the select list,
      <codeph>ORDER BY</codeph>, and <codeph>GROUP BY</codeph> clauses, you can use the <codeph>WITH</codeph>
      clause as an alternative to creating a view.
    </p>

    <p conref="../shared/impala_common.xml#common/complex_types_blurb"/>
    <p conref="../shared/impala_common.xml#common/complex_types_views"/>
    <p conref="../shared/impala_common.xml#common/complex_types_views_caveat"/>

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

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

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

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

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

<!-- TK: Elaborate on these, show queries and real output. -->

<codeblock>-- Create a view that is exactly the same as the underlying table.
create view v1 as select * from t1;

-- Create a view that includes only certain columns from the underlying table.
create view v2 as select c1, c3, c7 from t1;

-- Create a view that filters the values from the underlying table.
create view v3 as select distinct c1, c3, c7 from t1 where c1 is not null and c5 &gt; 0;

-- Create a view that that reorders and renames columns from the underlying table.
create view v4 as select c4 as last_name, c6 as address, c2 as birth_date from t1;

-- Create a view that runs functions to convert or transform certain columns.
create view v5 as select c1, cast(c3 as string) c3, concat(c4,c5) c5, trim(c6) c6, "Constant" c8 from t1;

-- Create a view that hides the complexity of a view query.
create view v6 as select t1.c1, t2.c2 from t1 join t2 on t1.id = t2.id;
</codeblock>

<!-- These examples show CREATE VIEW and corresponding DROP VIEW statements, with different combinations
     of qualified and unqualified names. -->

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

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

    <p>
      <xref href="impala_views.xml#views"/>, <xref href="impala_alter_view.xml#alter_view"/>,
      <xref href="impala_drop_view.xml#drop_view"/>
    </p>
  </conbody>
</concept>