impala/docs/topics/impala_select.xml

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

  <title>SELECT Statement</title>
  <titlealts audience="PDF"><navtitle>SELECT</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Reports"/>
      <data name="Category" value="Tables"/>
      <data name="Category" value="Data Analysts"/>
      <data name="Category" value="Developers"/>
      <!-- This is such an important statement, think if there are more applicable categories. -->
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">SELECT statement</indexterm>
      The <codeph>SELECT</codeph> statement performs queries, retrieving data from one or more tables and producing
      result sets consisting of rows and columns.
    </p>

    <p>
      The Impala <codeph><xref href="impala_insert.xml#insert">INSERT</xref></codeph> statement also typically ends
      with a <codeph>SELECT</codeph> statement, to define data to copy from one table to another.
    </p>

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

<codeblock>[WITH <i>name</i> AS (<i>select_expression</i>) [, ...] ]
SELECT
  [ALL | DISTINCT]
  [STRAIGHT_JOIN]
  <i>expression</i> [, <i>expression</i> ...]
FROM <i>table_reference</i> [, <i>table_reference</i> ...]
[[FULL | [LEFT | RIGHT] INNER | [LEFT | RIGHT] OUTER | [LEFT | RIGHT] SEMI | [LEFT | RIGHT] ANTI | CROSS]
  JOIN <i>table_reference</i>
  [ON <i>join_equality_clauses</i> | USING (<varname>col1</varname>[, <varname>col2</varname> ...]] ...
WHERE <i>conditions</i>
GROUP BY { <i>column</i> | <i>expression</i> [ASC | DESC] [NULLS FIRST | NULLS LAST] [, ...] }
HAVING <codeph>conditions</codeph>
GROUP BY { <i>column</i> | <i>expression</i> [ASC | DESC] [, ...] }
LIMIT <i>expression</i> [OFFSET <i>expression</i>]
[UNION [ALL] <i>select_statement</i>] ...]
</codeblock>

    <p>
      Impala <codeph>SELECT</codeph> queries support:
    </p>

    <ul>
      <li>
        SQL scalar data types: <codeph><xref href="impala_boolean.xml#boolean">BOOLEAN</xref></codeph>,
        <codeph><xref href="impala_tinyint.xml#tinyint">TINYINT</xref></codeph>,
        <codeph><xref href="impala_smallint.xml#smallint">SMALLINT</xref></codeph>,
        <codeph><xref href="impala_int.xml#int">INT</xref></codeph>,
        <codeph><xref href="impala_bigint.xml#bigint">BIGINT</xref></codeph>,
        <codeph><xref href="impala_decimal.xml#decimal">DECIMAL</xref></codeph>
        <codeph><xref href="impala_float.xml#float">FLOAT</xref></codeph>,
        <codeph><xref href="impala_double.xml#double">DOUBLE</xref></codeph>,
        <codeph><xref href="impala_timestamp.xml#timestamp">TIMESTAMP</xref></codeph>,
        <codeph><xref href="impala_string.xml#string">STRING</xref></codeph>,
        <codeph><xref href="impala_varchar.xml#varchar">VARCHAR</xref></codeph>,
        <codeph><xref href="impala_char.xml#char">CHAR</xref></codeph>.
      </li>

<!-- To do: Consider promoting 'querying complex types' to its own subtopic or pseudo-heading. -->
      <li rev="2.3.0">
        The complex data types <codeph>ARRAY</codeph>, <codeph>STRUCT</codeph>, and <codeph>MAP</codeph>,
        are available in <keyword keyref="impala23_full"/> and higher.
        Queries involving these types typically involve special qualified names
        using dot notation for referring to the complex column fields,
        and join clauses for bringing the complex columns into the result set.
        See <xref href="impala_complex_types.xml#complex_types"/> for details.
      </li>

      <li rev="1.1">
        An optional <xref href="impala_with.xml#with"><codeph>WITH</codeph> clause</xref> before the
        <codeph>SELECT</codeph> keyword, to define a subquery whose name or column names can be referenced from
        later in the main query. This clause lets you abstract repeated clauses, such as aggregation functions,
        that are referenced multiple times in the same query.
      </li>

      <li>
        By default, one <codeph>DISTINCT</codeph> clause per query. See <xref href="impala_distinct.xml#distinct"/>
        for details. See <xref href="impala_appx_count_distinct.xml#appx_count_distinct"/> for a query option to
        allow multiple <codeph>COUNT(DISTINCT)</codeph> impressions in the same query.
      </li>

      <li>
        Subqueries in a <codeph>FROM</codeph> clause. In <keyword keyref="impala20_full"/> and higher,
        subqueries can also go in the <codeph>WHERE</codeph> clause, for example with the
        <codeph>IN()</codeph>, <codeph>EXISTS</codeph>, and <codeph>NOT EXISTS</codeph> operators.
      </li>

      <li>
        <codeph>WHERE</codeph>, <codeph>GROUP BY</codeph>, <codeph>HAVING</codeph> clauses.
      </li>

      <li rev="obwl">
        <codeph><xref href="impala_order_by.xml#order_by">ORDER BY</xref></codeph>. Prior to Impala 1.4.0, Impala
        required that queries using an <codeph>ORDER BY</codeph> clause also include a
        <codeph><xref href="impala_limit.xml#limit">LIMIT</xref></codeph> clause. In Impala 1.4.0 and higher, this
        restriction is lifted; sort operations that would exceed the Impala memory limit automatically use a
        temporary disk work area to perform the sort.
      </li>

      <li>
        <p conref="../shared/impala_common.xml#common/join_types"/>
        <p>
          See <xref href="impala_joins.xml#joins"/> for details and examples of join queries.
        </p>
      </li>

      <li>
        <codeph>UNION ALL</codeph>.
      </li>

      <li>
        <codeph>LIMIT</codeph>.
      </li>

      <li>
        External tables.
      </li>

      <li>
        Relational operators such as greater than, less than, or equal to.
      </li>

      <li>
        Arithmetic operators such as addition or subtraction.
      </li>

      <li>
        Logical/Boolean operators <codeph>AND</codeph>, <codeph>OR</codeph>, and <codeph>NOT</codeph>. Impala does
        not support the corresponding symbols <codeph>&amp;&amp;</codeph>, <codeph>||</codeph>, and
        <codeph>!</codeph>.
      </li>

      <li>
        Common SQL built-in functions such as <codeph>COUNT</codeph>, <codeph>SUM</codeph>, <codeph>CAST</codeph>,
        <codeph>LIKE</codeph>, <codeph>IN</codeph>, <codeph>BETWEEN</codeph>, and <codeph>COALESCE</codeph>. Impala
        specifically supports built-ins described in <xref href="impala_functions.xml#builtins"/>.
      </li>
    </ul>

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

    <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/s3_blurb"/>
    <p conref="../shared/impala_common.xml#common/s3_block_splitting"/>

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

    <p conref="../shared/impala_common.xml#common/permissions_blurb"/>
    <p rev="CDH-19187">
      The user ID that the <cmdname>impalad</cmdname> daemon runs under,
      typically the <codeph>impala</codeph> user, must have read
      permissions for the files in all applicable directories in all source tables,
      and read and execute permissions for the relevant data directories.
      (A <codeph>SELECT</codeph> operation could read files from multiple different HDFS directories
      if the source table is partitioned.)
      If a query attempts to read a data file and is unable to because of an HDFS permission error,
      the query halts and does not return any further results.
    </p>

    <p outputclass="toc"/>

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

    <p>
      The <codeph>SELECT</codeph> syntax is so extensive that it forms its own category of statements: queries. The
      other major classifications of SQL statements are data definition language (see
      <xref href="impala_ddl.xml#ddl"/>) and data manipulation language (see <xref href="impala_dml.xml#dml"/>).
    </p>

    <p>
      Because the focus of Impala is on fast queries with interactive response times over huge data sets, query
      performance and scalability are important considerations. See
      <xref href="impala_performance.xml#performance"/> and <xref href="impala_scalability.xml#scalability"/> for
      details.
    </p>
  </conbody>

  <concept id="where" audience="hidden">

<!-- WHERE hidden for the moment until there's the chance to add some reasonably comprehensive content

     and make it its own file. -->

    <title>WHERE Clause</title>

    <conbody>

      <p/>
    </conbody>
  </concept>
</concept>