<para>

<firstterm>Aggregate functions</firstterm> allow the generation of simple

statistics about the values ofparticular columns over the selected set

statistics about the values ofgiven expressions over the selected set

of rows.

<!--

See also <xref linkend="sql" endterm="aggregates-tutorial"> and

<title>Aggregate Functions</title>

<tgroup cols="4">

<thead>

<row>

<entry>Function</entry>

<entry>Returns</entry>

<entry>Description</entry>

<entry>Example</entry>

<entry>Notes</entry>

</row>

</thead>

<tbody>

<row>

<entry>COUNT(*)</entry>

<entry></entry>

</row>

<row>

<entry>COUNT(<replaceable class="parameter">column-name</replaceable>)</entry>

<entry>COUNT(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>int4</entry>

<entry>Counts the selected rows for which the value of <replaceable class="parameter">column-name</replaceable> is not NULL.</entry>

<entry>Counts the selected rows for which the value of

<replaceable class="parameter">expression</replaceable> is not

NULL.</entry>

<entry>COUNT(age)</entry>

<entry></entry>

</row>

<row>

<entry>SUM(<replaceable class="parameter">column-name</replaceable>)</entry>

<entry>Same as the data type of the column being summed.</entry>

<entry>Finds the total obtained by adding the values of <replaceable class="parameter">column-name</replaceable> across all selected rows.</entry>

<entry>SUM(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>Depends on the data type being summed.</entry>

<entry>Finds the total obtained by adding the values of <replaceable class="parameter">expression</replaceable> across all selected rows.</entry>

<entry>SUM(hours)</entry>

<entry>Summation is supported on the following data types: int8, int4, int2, float4, float8, money, interval, numeric</entry>

<entry>Summation is supported on the following data types: int8, int4,

int2, float4, float8, money, interval, numeric. The result is numeric

for any integer type, float8 for either float4 or float8 input,

otherwise the same as the input data type.</entry>

</row>

<row>

<entry>MAX(<replaceable class="parameter">column-name</replaceable>)</entry>

<entry>Same as the data type of thecolumn whose maximum value is sought.</entry>

<entry>The maximum value of <replaceable class="parameter">column-name</replaceable> across all selected rows.</entry>

<entry>MAX(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>Same as the data type of theinput expression.</entry>

<entry>The maximum value of <replaceable class="parameter">expression</replaceable> across all selected rows.</entry>

<entry>MAX(age)</entry>

<entry>Finding the maximum value is supported on the following data types: int8, int4, int2, float4, float8, date, time, timetz, money, timestamp, interval, text, numeric.</entry>

</row>

<row>

<entry>MIN(<replaceable class="parameter">column-name</replaceable>)</entry>

<entry>same as the data type of thecolumn whose minimum value is sought.</entry>

<entry>The minimum value of <replaceable class="parameter">column-name</replaceable> across all selected rows.</entry>

<entry>MIN(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>Same as the data type of theinput expression.</entry>

<entry>The minimum value of <replaceable class="parameter">expression</replaceable> across all selected rows.</entry>

<entry>MIN(age)</entry>

<entry>Finding the minimum value is supported on the following data types: int8, int4, int2, float4, float8, date, time, timetz, money, timestamp, interval, text, numeric.</entry>

</row>

<row>

<entry>AVG(<replaceable class="parameter">column-name</replaceable>)</entry>

<entry>Same as the data type of the column being averaged.</entry>

<entry>The average (mean) of the values in the given column across all selected rows.</entry>

<entry>AVG(age)</entry>

<entry>Finding the mean value is supported on the following data types: int8, int4, int2, float4, float8, money, interval, numeric. Note that as the return type is the same as that of the data being averaged, using AVG() on discrete data will give a rounded result.</entry>

<entry>AVG(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>Depends on the data type being averaged.</entry>

<entry>The average (mean) of the given values across all selected rows.</entry>

<entry>AVG(age+1)</entry>

<entry>Finding the mean value is supported on the following data

types: int8, int4, int2, float4, float8, interval, numeric. The

result is numeric for any integer type, float8 for either float4 or

float8 input, otherwise the same as the input data type.</entry>

</row>

<row>

<entry>VARIANCE(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>Depends on the input data type.</entry>

<entry>The sample variance of the given values.</entry>

<entry>VARIANCE(reading)</entry>

<entry>Finding the variance is supported on the following data

types: int8, int4, int2, float4, float8, numeric. The result is

float8 for float4 or float8 input, otherwise numeric.</entry>

</row>

<row>

<entry>STDDEV(<replaceable class="parameter">expression</replaceable>)</entry>

<entry>Depends on the input data type.</entry>

<entry>The sample standard deviation of the given values.</entry>

<entry>STDDEV(reading)</entry>

<entry>Finding the standard deviation is supported on the following

data types: int8, int4, int2, float4, float8, numeric. The result is

float8 for float4 or float8 input, otherwise numeric.</entry>

</row>

</tbody>

</tgroup>

</table>

</para>

<para>

It should be noted that except for COUNT, these functions return NULL

when no rows are selected. In particular, SUM of no rows returns NULL,

not zero as one might expect.

</para>

</sect1>

</chapter>