docs/topics/impala_with.xml

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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept rev="1.1" id="with">

  <title>WITH Clause</title>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      A clause that can be added before a <codeph>SELECT</codeph> statement, to define aliases for complicated
      expressions that are referenced multiple times within the body of the <codeph>SELECT</codeph>. Similar to
      <codeph>CREATE VIEW</codeph>, except that the table and column names defined in the <codeph>WITH</codeph>
      clause do not persist after the query finishes, and do not conflict with names used in actual tables or
      views. Also known as <q>subquery factoring</q>.
    </p>

    <p>
      You can rewrite a query using subqueries to work the same as with the <codeph>WITH</codeph> clause. The
      purposes of the <codeph>WITH</codeph> clause are:
    </p>

    <ul>
      <li>
        Convenience and ease of maintenance from less repetition with the body of the query. Typically used with
        queries involving <codeph>UNION</codeph>, joins, or aggregation functions where the similar complicated
        expressions are referenced multiple times.
      </li>

      <li>
        SQL code that is easier to read and understand by abstracting the most complex part of the query into a
        separate block.
      </li>

      <li>
        Improved compatibility with SQL from other database systems that support the same clause (primarily Oracle
        Database).
        <note>
          <p>
            The Impala <codeph>WITH</codeph> clause does not support recursive queries in the
            <codeph>WITH</codeph>, which is supported in some other database systems.
          </p>
        </note>
      </li>
    </ul>

    <p conref="../shared/impala_common.xml#common/sql1999"/>

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

<codeblock>-- Define 2 subqueries that can be referenced from the body of a longer query.
with t1 as (select 1), t2 as (select 2) insert into tab select * from t1 union all select * from t2;

-- Define one subquery at the outer level, and another at the inner level as part of the
-- initial stage of the UNION ALL query.
with t1 as (select 1) (with t2 as (select 2) select * from t2) union all select * from t1;</codeblock>
  </conbody>
</concept>