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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> [, ...] } |
| 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> |
| </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> |
| 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/column_ordinals"/> |
| </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>&&</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> |
| |
| <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. |
| </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=""> |
| 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> |
