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These are refugees from doc_prototype. They can be rendered with the DITA Open Toolkit version 2.3.3 by: /tmp/dita-ot-2.3.3/bin/dita \ -i impala.ditamap \ -f html5 \ -o $(mktemp -d) \ -filter impala_html.ditaval Change-Id: I8861e99adc446f659a04463ca78c79200669484f Reviewed-on: http://gerrit.cloudera.org:8080/5014 Reviewed-by: John Russell <jrussell@cloudera.com> Tested-by: John Russell <jrussell@cloudera.com>
199 lines
8.2 KiB
XML
199 lines
8.2 KiB
XML
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="max">
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<title>MAX Function</title>
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<titlealts audience="PDF"><navtitle>MAX</navtitle></titlealts>
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<prolog>
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<metadata>
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<data name="Category" value="Impala"/>
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<data name="Category" value="SQL"/>
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<data name="Category" value="Impala Functions"/>
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<data name="Category" value="Analytic Functions"/>
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<data name="Category" value="Aggregate Functions"/>
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<data name="Category" value="Querying"/>
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<data name="Category" value="Developers"/>
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<data name="Category" value="Data Analysts"/>
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</metadata>
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</prolog>
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<conbody>
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<p>
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<indexterm audience="Cloudera">max() function</indexterm>
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An aggregate function that returns the maximum value from a set of numbers. Opposite of the
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<codeph>MIN</codeph> function. Its single argument can be numeric column, or the numeric result of a function
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or expression applied to the column value. Rows with a <codeph>NULL</codeph> value for the specified column
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are ignored. If the table is empty, or all the values supplied to <codeph>MAX</codeph> are
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<codeph>NULL</codeph>, <codeph>MAX</codeph> returns <codeph>NULL</codeph>.
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</p>
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<p conref="../shared/impala_common.xml#common/syntax_blurb"/>
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<codeblock>MAX([DISTINCT | ALL] <varname>expression</varname>) [OVER (<varname>analytic_clause</varname>)]</codeblock>
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<p>
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When the query contains a <codeph>GROUP BY</codeph> clause, returns one value for each combination of
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grouping values.
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</p>
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<p conref="../shared/impala_common.xml#common/restrictions_sliding_window"/>
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<p conref="../shared/impala_common.xml#common/return_type_same_except_string"/>
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<p conref="../shared/impala_common.xml#common/usage_notes_blurb"/>
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<p conref="../shared/impala_common.xml#common/partition_key_optimization"/>
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<p conref="../shared/impala_common.xml#common/complex_types_blurb"/>
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<p conref="../shared/impala_common.xml#common/complex_types_aggregation_explanation"/>
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<p conref="../shared/impala_common.xml#common/complex_types_aggregation_example"/>
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<p conref="../shared/impala_common.xml#common/example_blurb"/>
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<codeblock>-- Find the largest value for this column in the table.
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select max(c1) from t1;
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-- Find the largest value for this column from a subset of the table.
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select max(c1) from t1 where month = 'January' and year = '2013';
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-- Find the largest value from a set of numeric function results.
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select max(length(s)) from t1;
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-- Can also be used in combination with DISTINCT and/or GROUP BY.
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-- Return more than one result.
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select month, year, max(purchase_price) from store_stats group by month, year;
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-- Filter the input to eliminate duplicates before performing the calculation.
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select max(distinct x) from t1;
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</codeblock>
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<p rev="2.0.0">
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The following examples show how to use <codeph>MAX()</codeph> in an analytic context. They use a table
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containing integers from 1 to 10. Notice how the <codeph>MAX()</codeph> is reported for each input value, as
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opposed to the <codeph>GROUP BY</codeph> clause which condenses the result set.
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<codeblock>select x, property, max(x) over (partition by property) as max from int_t where property in ('odd','even');
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+----+----------+-----+
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| x | property | max |
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+----+----------+-----+
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| 2 | even | 10 |
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| 4 | even | 10 |
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| 6 | even | 10 |
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| 8 | even | 10 |
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| 10 | even | 10 |
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| 1 | odd | 9 |
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| 3 | odd | 9 |
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| 5 | odd | 9 |
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| 7 | odd | 9 |
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| 9 | odd | 9 |
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+----+----------+-----+
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</codeblock>
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Adding an <codeph>ORDER BY</codeph> clause lets you experiment with results that are cumulative or apply to a moving
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set of rows (the <q>window</q>). The following examples use <codeph>MAX()</codeph> in an analytic context
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(that is, with an <codeph>OVER()</codeph> clause) to display the smallest value of <codeph>X</codeph>
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encountered up to each row in the result set. The examples use two columns in the <codeph>ORDER BY</codeph>
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clause to produce a sequence of values that rises and falls, to illustrate how the <codeph>MAX()</codeph>
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result only increases or stays the same throughout each partition within the result set.
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The basic <codeph>ORDER BY x</codeph> clause implicitly
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activates a window clause of <codeph>RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW</codeph>,
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which is effectively the same as <codeph>ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW</codeph>,
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therefore all of these examples produce the same results:
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<codeblock>select x, property,
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max(x) <b>over (order by property, x desc)</b> as 'maximum to this point'
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from int_t where property in ('prime','square');
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+---+----------+-----------------------+
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| x | property | maximum to this point |
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+---+----------+-----------------------+
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| 7 | prime | 7 |
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| 5 | prime | 7 |
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| 3 | prime | 7 |
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| 2 | prime | 7 |
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| 9 | square | 9 |
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| 4 | square | 9 |
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| 1 | square | 9 |
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+---+----------+-----------------------+
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select x, property,
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max(x) over
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(
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<b>order by property, x desc</b>
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<b>rows between unbounded preceding and current row</b>
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) as 'maximum to this point'
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from int_t where property in ('prime','square');
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+---+----------+-----------------------+
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| x | property | maximum to this point |
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+---+----------+-----------------------+
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| 7 | prime | 7 |
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| 5 | prime | 7 |
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| 3 | prime | 7 |
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| 2 | prime | 7 |
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| 9 | square | 9 |
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| 4 | square | 9 |
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| 1 | square | 9 |
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+---+----------+-----------------------+
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select x, property,
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max(x) over
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(
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<b>order by property, x desc</b>
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<b>range between unbounded preceding and current row</b>
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) as 'maximum to this point'
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from int_t where property in ('prime','square');
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+---+----------+-----------------------+
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| x | property | maximum to this point |
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+---+----------+-----------------------+
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| 7 | prime | 7 |
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| 5 | prime | 7 |
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| 3 | prime | 7 |
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| 2 | prime | 7 |
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| 9 | square | 9 |
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| 4 | square | 9 |
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| 1 | square | 9 |
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+---+----------+-----------------------+
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</codeblock>
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The following examples show how to construct a moving window, with a running maximum taking into account all rows before
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and 1 row after the current row.
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Because of a restriction in the Impala <codeph>RANGE</codeph> syntax, this type of
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moving window is possible with the <codeph>ROWS BETWEEN</codeph> clause but not the <codeph>RANGE BETWEEN</codeph> clause.
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Because of an extra Impala restriction on the <codeph>MAX()</codeph> and <codeph>MIN()</codeph> functions in an
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analytic context, the lower bound must be <codeph>UNBOUNDED PRECEDING</codeph>.
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<codeblock>select x, property,
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max(x) over
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(
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<b>order by property, x</b>
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<b>rows between unbounded preceding and 1 following</b>
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) as 'local maximum'
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from int_t where property in ('prime','square');
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+---+----------+---------------+
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| x | property | local maximum |
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+---+----------+---------------+
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| 2 | prime | 3 |
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| 3 | prime | 5 |
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| 5 | prime | 7 |
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| 7 | prime | 7 |
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| 1 | square | 7 |
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| 4 | square | 9 |
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| 9 | square | 9 |
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+---+----------+---------------+
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-- Doesn't work because of syntax restriction on RANGE clause.
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select x, property,
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max(x) over
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(
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<b>order by property, x</b>
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<b>range between unbounded preceding and 1 following</b>
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) as 'local maximum'
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from int_t where property in ('prime','square');
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ERROR: AnalysisException: RANGE is only supported with both the lower and upper bounds UNBOUNDED or one UNBOUNDED and the other CURRENT ROW.
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</codeblock>
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</p>
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<p conref="../shared/impala_common.xml#common/related_info"/>
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<p>
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<xref href="impala_analytic_functions.xml#analytic_functions"/>, <xref href="impala_min.xml#min"/>,
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<xref href="impala_avg.xml#avg"/>
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</p>
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</conbody>
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</concept>
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