impala/docs/topics/impala_max.xml

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

  <title>MAX Function</title>
  <titlealts audience="PDF"><navtitle>MAX</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="SQL"/>
      <data name="Category" value="Impala Functions"/>
      <data name="Category" value="Analytic Functions"/>
      <data name="Category" value="Aggregate Functions"/>
      <data name="Category" value="Querying"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      <indexterm audience="hidden">max() function</indexterm>
      An aggregate function that returns the maximum value from a set of numbers. Opposite of the
      <codeph>MIN</codeph> function. Its single argument can be numeric column, or the numeric result of a function
      or expression applied to the column value. Rows with a <codeph>NULL</codeph> value for the specified column
      are ignored. If the table is empty, or all the values supplied to <codeph>MAX</codeph> are
      <codeph>NULL</codeph>, <codeph>MAX</codeph> returns <codeph>NULL</codeph>.
    </p>

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

<codeblock>MAX([DISTINCT | ALL] <varname>expression</varname>) [OVER (<varname>analytic_clause</varname>)]</codeblock>

    <p>
      When the query contains a <codeph>GROUP BY</codeph> clause, returns one value for each combination of
      grouping values.
    </p>

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

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

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

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

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

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

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

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

<codeblock>-- Find the largest value for this column in the table.
select max(c1) from t1;
-- Find the largest value for this column from a subset of the table.
select max(c1) from t1 where month = 'January' and year = '2013';
-- Find the largest value from a set of numeric function results.
select max(length(s)) from t1;
-- Can also be used in combination with DISTINCT and/or GROUP BY.
-- Return more than one result.
select month, year, max(purchase_price) from store_stats group by month, year;
-- Filter the input to eliminate duplicates before performing the calculation.
select max(distinct x) from t1;
</codeblock>

    <p rev="2.0.0">
      The following examples show how to use <codeph>MAX()</codeph> in an analytic context. They use a table
      containing integers from 1 to 10. Notice how the <codeph>MAX()</codeph> is reported for each input value, as
      opposed to the <codeph>GROUP BY</codeph> clause which condenses the result set.
<codeblock>select x, property, max(x) over (partition by property) as max from int_t where property in ('odd','even');
+----+----------+-----+
| x  | property | max |
+----+----------+-----+
| 2  | even     | 10  |
| 4  | even     | 10  |
| 6  | even     | 10  |
| 8  | even     | 10  |
| 10 | even     | 10  |
| 1  | odd      | 9   |
| 3  | odd      | 9   |
| 5  | odd      | 9   |
| 7  | odd      | 9   |
| 9  | odd      | 9   |
+----+----------+-----+
</codeblock>

Adding an <codeph>ORDER BY</codeph> clause lets you experiment with results that are cumulative or apply to a moving
set of rows (the <q>window</q>). The following examples use <codeph>MAX()</codeph> in an analytic context
(that is, with an <codeph>OVER()</codeph> clause) to display the smallest value of <codeph>X</codeph>
encountered up to each row in the result set. The examples use two columns in the <codeph>ORDER BY</codeph>
clause to produce a sequence of values that rises and falls, to illustrate how the <codeph>MAX()</codeph>
result only increases or stays the same throughout each partition within the result set.
The basic <codeph>ORDER BY x</codeph> clause implicitly
activates a window clause of <codeph>RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW</codeph>,
which is effectively the same as <codeph>ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW</codeph>,
therefore all of these examples produce the same results:

<codeblock>select x, property,
  max(x) <b>over (order by property, x desc)</b> as 'maximum to this point'
from int_t where property in ('prime','square');
+---+----------+-----------------------+
| x | property | maximum to this point |
+---+----------+-----------------------+
| 7 | prime    | 7                     |
| 5 | prime    | 7                     |
| 3 | prime    | 7                     |
| 2 | prime    | 7                     |
| 9 | square   | 9                     |
| 4 | square   | 9                     |
| 1 | square   | 9                     |
+---+----------+-----------------------+

select x, property,
  max(x) over
  (
    <b>order by property, x desc</b>
    <b>rows between unbounded preceding and current row</b>
  ) as 'maximum to this point'
from int_t where property in ('prime','square');
+---+----------+-----------------------+
| x | property | maximum to this point |
+---+----------+-----------------------+
| 7 | prime    | 7                     |
| 5 | prime    | 7                     |
| 3 | prime    | 7                     |
| 2 | prime    | 7                     |
| 9 | square   | 9                     |
| 4 | square   | 9                     |
| 1 | square   | 9                     |
+---+----------+-----------------------+

select x, property,
  max(x) over
  (
    <b>order by property, x desc</b>
    <b>range between unbounded preceding and current row</b>
  ) as 'maximum to this point'
from int_t where property in ('prime','square');
+---+----------+-----------------------+
| x | property | maximum to this point |
+---+----------+-----------------------+
| 7 | prime    | 7                     |
| 5 | prime    | 7                     |
| 3 | prime    | 7                     |
| 2 | prime    | 7                     |
| 9 | square   | 9                     |
| 4 | square   | 9                     |
| 1 | square   | 9                     |
+---+----------+-----------------------+
</codeblock>

The following examples show how to construct a moving window, with a running maximum taking into account all rows before
and 1 row after the current row.
Because of a restriction in the Impala <codeph>RANGE</codeph> syntax, this type of
moving window is possible with the <codeph>ROWS BETWEEN</codeph> clause but not the <codeph>RANGE BETWEEN</codeph> clause.
Because of an extra Impala restriction on the <codeph>MAX()</codeph> and <codeph>MIN()</codeph> functions in an
analytic context, the lower bound must be <codeph>UNBOUNDED PRECEDING</codeph>.
<codeblock>select x, property,
  max(x) over
  (
    <b>order by property, x</b>
    <b>rows between unbounded preceding and 1 following</b>
  ) as 'local maximum'
from int_t where property in ('prime','square');
+---+----------+---------------+
| x | property | local maximum |
+---+----------+---------------+
| 2 | prime    | 3             |
| 3 | prime    | 5             |
| 5 | prime    | 7             |
| 7 | prime    | 7             |
| 1 | square   | 7             |
| 4 | square   | 9             |
| 9 | square   | 9             |
+---+----------+---------------+

-- Doesn't work because of syntax restriction on RANGE clause.
select x, property,
  max(x) over
  (
    <b>order by property, x</b>
    <b>range between unbounded preceding and 1 following</b>
  ) as 'local maximum'
from int_t where property in ('prime','square');
ERROR: AnalysisException: RANGE is only supported with both the lower and upper bounds UNBOUNDED or one UNBOUNDED and the other CURRENT ROW.
</codeblock>
    </p>

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

    <p>
      <xref href="impala_analytic_functions.xml#analytic_functions"/>, <xref href="impala_min.xml#min"/>,
      <xref href="impala_avg.xml#avg"/>
    </p>
  </conbody>
</concept>