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

<concept id="avg"> | |

<title>AVG Function</title> | |

<titlealts audience="PDF"><navtitle>AVG</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">avg() function</indexterm> | |

An aggregate function that returns the average value from a set of numbers or <codeph>TIMESTAMP</codeph> values. | |

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>AVG</codeph> are <codeph>NULL</codeph>, <codeph>AVG</codeph> returns | |

<codeph>NULL</codeph>. | |

</p> | |

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

<codeblock>AVG([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> | |

<b>Return type:</b> <codeph>DOUBLE</codeph> for numeric values; <codeph>TIMESTAMP</codeph> for | |

<codeph>TIMESTAMP</codeph> values | |

</p> | |

<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>-- Average all the non-NULL values in a column. | |

insert overwrite avg_t values (2),(4),(6),(null),(null); | |

-- The average of the above values is 4: (2+4+6) / 3. The 2 NULL values are ignored. | |

select avg(x) from avg_t; | |

-- Average only certain values from the column. | |

select avg(x) from t1 where month = 'January' and year = '2013'; | |

-- Apply a calculation to the value of the column before averaging. | |

select avg(x/3) from t1; | |

-- Apply a function to the value of the column before averaging. | |

-- Here we are substituting a value of 0 for all NULLs in the column, | |

-- so that those rows do factor into the return value. | |

select avg(isnull(x,0)) from t1; | |

-- Apply some number-returning function to a string column and average the results. | |

-- If column s contains any NULLs, length(s) also returns NULL and those rows are ignored. | |

select avg(length(s)) from t1; | |

-- Can also be used in combination with DISTINCT and/or GROUP BY. | |

-- Return more than one result. | |

select month, year, avg(page_visits) from web_stats group by month, year; | |

-- Filter the input to eliminate duplicates before performing the calculation. | |

select avg(distinct x) from t1; | |

-- Filter the output after performing the calculation. | |

select avg(x) from t1 group by y having avg(x) between 1 and 20; | |

</codeblock> | |

<p rev="2.0.0"> | |

The following examples show how to use <codeph>AVG()</codeph> in an analytic context. They use a table | |

containing integers from 1 to 10. Notice how the <codeph>AVG()</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, avg(x) over (partition by property) as avg from int_t where property in ('odd','even'); | |

+----+----------+-----+ | |

| x | property | avg | | |

+----+----------+-----+ | |

| 2 | even | 6 | | |

| 4 | even | 6 | | |

| 6 | even | 6 | | |

| 8 | even | 6 | | |

| 10 | even | 6 | | |

| 1 | odd | 5 | | |

| 3 | odd | 5 | | |

| 5 | odd | 5 | | |

| 7 | odd | 5 | | |

| 9 | odd | 5 | | |

+----+----------+-----+ | |

</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>AVG()</codeph> in an analytic context | |

(that is, with an <codeph>OVER()</codeph> clause) to produce a running average of all the even values, | |

then a running average of all the odd values. 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, | |

avg(x) over (partition by property <b>order by x</b>) as 'cumulative average' | |

from int_t where property in ('odd','even'); | |

+----+----------+--------------------+ | |

| x | property | cumulative average | | |

+----+----------+--------------------+ | |

| 2 | even | 2 | | |

| 4 | even | 3 | | |

| 6 | even | 4 | | |

| 8 | even | 5 | | |

| 10 | even | 6 | | |

| 1 | odd | 1 | | |

| 3 | odd | 2 | | |

| 5 | odd | 3 | | |

| 7 | odd | 4 | | |

| 9 | odd | 5 | | |

+----+----------+--------------------+ | |

select x, property, | |

avg(x) over | |

( | |

partition by property | |

<b>order by x</b> | |

<b>range between unbounded preceding and current row</b> | |

) as 'cumulative average' | |

from int_t where property in ('odd','even'); | |

+----+----------+--------------------+ | |

| x | property | cumulative average | | |

+----+----------+--------------------+ | |

| 2 | even | 2 | | |

| 4 | even | 3 | | |

| 6 | even | 4 | | |

| 8 | even | 5 | | |

| 10 | even | 6 | | |

| 1 | odd | 1 | | |

| 3 | odd | 2 | | |

| 5 | odd | 3 | | |

| 7 | odd | 4 | | |

| 9 | odd | 5 | | |

+----+----------+--------------------+ | |

select x, property, | |

avg(x) over | |

( | |

partition by property | |

<b>order by x</b> | |

<b>rows between unbounded preceding and current row</b> | |

) as 'cumulative average' | |

from int_t where property in ('odd','even'); | |

+----+----------+--------------------+ | |

| x | property | cumulative average | | |

+----+----------+--------------------+ | |

| 2 | even | 2 | | |

| 4 | even | 3 | | |

| 6 | even | 4 | | |

| 8 | even | 5 | | |

| 10 | even | 6 | | |

| 1 | odd | 1 | | |

| 3 | odd | 2 | | |

| 5 | odd | 3 | | |

| 7 | odd | 4 | | |

| 9 | odd | 5 | | |

+----+----------+--------------------+ | |

</codeblock> | |

The following examples show how to construct a moving window, with a running average taking into account 1 row before | |

and 1 row after the current row, within the same partition (all the even values or all the odd values). | |

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: | |

<codeblock>select x, property, | |

avg(x) over | |

( | |

partition by property | |

<b>order by x</b> | |

<b>rows between 1 preceding and 1 following</b> | |

) as 'moving average' | |

from int_t where property in ('odd','even'); | |

+----+----------+----------------+ | |

| x | property | moving average | | |

+----+----------+----------------+ | |

| 2 | even | 3 | | |

| 4 | even | 4 | | |

| 6 | even | 6 | | |

| 8 | even | 8 | | |

| 10 | even | 9 | | |

| 1 | odd | 2 | | |

| 3 | odd | 3 | | |

| 5 | odd | 5 | | |

| 7 | odd | 7 | | |

| 9 | odd | 8 | | |

+----+----------+----------------+ | |

-- Doesn't work because of syntax restriction on RANGE clause. | |

select x, property, | |

avg(x) over | |

( | |

partition by property | |

<b>order by x</b> | |

<b>range between 1 preceding and 1 following</b> | |

) as 'moving average' | |

from int_t where property in ('odd','even'); | |

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/restrictions_blurb"/> | |

<!-- This conref appears under SUM(), AVG(), FLOAT, and DOUBLE topics. --> | |

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

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

<p> | |

<xref href="impala_analytic_functions.xml#analytic_functions"/>, <xref href="impala_max.xml#max"/>, | |

<xref href="impala_min.xml#min"/> | |

</p> | |

</conbody> | |

</concept> |