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| <h1 class="title topictitle1" id="ariaid-title1">WITH Clause</h1> |
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| <div class="body conbody"> |
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| <p class="p"> |
| A clause that can be added before a <code class="ph codeph">SELECT</code> statement, to define aliases for complicated |
| expressions that are referenced multiple times within the body of the <code class="ph codeph">SELECT</code>. Similar to |
| <code class="ph codeph">CREATE VIEW</code>, except that the table and column names defined in the <code class="ph codeph">WITH</code> |
| clause do not persist after the query finishes, and do not conflict with names used in actual tables or |
| views. Also known as <span class="q">"subquery factoring"</span>. |
| </p> |
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| <p class="p"> |
| You can rewrite a query using subqueries to work the same as with the <code class="ph codeph">WITH</code> clause. The |
| purposes of the <code class="ph codeph">WITH</code> clause are: |
| </p> |
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| <ul class="ul"> |
| <li class="li"> |
| Convenience and ease of maintenance from less repetition with the body of the query. Typically used with |
| queries involving <code class="ph codeph">UNION</code>, joins, or aggregation functions where the similar complicated |
| expressions are referenced multiple times. |
| </li> |
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| <li class="li"> |
| SQL code that is easier to read and understand by abstracting the most complex part of the query into a |
| separate block. |
| </li> |
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| <li class="li"> |
| Improved compatibility with SQL from other database systems that support the same clause (primarily Oracle |
| Database). |
| <div class="note note"><span class="notetitle">Note:</span> |
| <p class="p"> |
| The Impala <code class="ph codeph">WITH</code> clause does not support recursive queries in the |
| <code class="ph codeph">WITH</code>, which is supported in some other database systems. |
| </p> |
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| </div> |
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| </li> |
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| </ul> |
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| <p class="p"> |
| <strong class="ph b">Standards compliance:</strong> Introduced in |
| <a class="xref" href="http://en.wikipedia.org/wiki/SQL:1999" target="_blank">SQL:1999</a>. |
| </p> |
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| <p class="p"> |
| <strong class="ph b">Examples:</strong> |
| </p> |
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| <pre class="pre codeblock"><code>-- 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;</code></pre> |
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| <div class="related-links"> |
| <div class="familylinks"> |
| <div class="parentlink"><strong>Parent topic:</strong> <a class="link" href="../topics/impala_select.html">SELECT Statement</a></div> |
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