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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>
<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. -->
<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.
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 conref="../shared/impala_common.xml#common/syntax_blurb"/>
<codeblock>[WITH <i>name</i> AS (<i>select_expression</i>) [, ...] ]
<i>expression</i> [, <i>expression</i> ...]
FROM <i>table_reference</i> [, <i>table_reference</i> ...]
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> [, ...] }
HAVING <codeph>conditions</codeph>
ORDER BY { <i>column</i> | <i>expression</i> [ASC | DESC] [NULLS FIRST | NULLS LAST] [, ...] }
LIMIT <i>expression</i> [OFFSET <i>expression</i>]
[UNION [ALL] <i>select_statement</i>] ...]
table_reference := { <varname>table_name</varname> | (<varname>subquery</varname>) }
<ph rev="IMPALA-5309">[ TABLESAMPLE SYSTEM(<varname>percentage</varname>) [REPEATABLE(<varname>seed</varname>)] ]</ph>
Impala <codeph>SELECT</codeph> queries support:
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>.
<!-- 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 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.
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.
<codeph>WHERE</codeph>, <codeph>GROUP BY</codeph>, <codeph>HAVING</codeph> clauses.
<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.
<p conref="../shared/impala_common.xml#common/column_ordinals"/>
<p conref="../shared/impala_common.xml#common/join_types"/>
See <xref href="impala_joins.xml#joins"/> for details and examples of join queries.
<codeph>UNION ALL</codeph>.
External tables.
Relational operators such as greater than, less than, or equal to.
Arithmetic operators such as addition or subtraction.
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
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 rev="IMPALA-5309">
In <keyword keyref="impala29_full"/> and higher, an optional <codeph>TABLESAMPLE</codeph>
clause immediately after a table reference, to specify that the query only processes a
specified percentage of the table data. See <xref keyref="tablesample"/> for details.
<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="">
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 outputclass="toc"/>
<p conref="../shared/impala_common.xml#common/related_info"/>
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"/>).
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