Custom Aggregate Functions

Descriptions and examples of custom CGE aggregate functions.

CGE supports the following custom aggregate functions:

variance
standard deviation
geometric mean
mode
median

Table 1. Custom Aggregate Functions
Function	Purpose
variance	Returns the variance of an expression.
standard deviation	Calculates the standard deviation of a set of numeric values. Requires at least two values.
geometric mean	Calculates the `n`th root of the product of the numbers, where `n` is the count of numbers.
mode	Returns the most frequently occurring number in a group of supplied arguments.
median	Calculate the median, which is the value separating the higher half of a data sample, a population, or a probability distribution, from the lower half.

Examples

variance

SELECT ?p (AGG<http://www.dotnetrdf.org/leviathan#variance>(?o) AS ?RESULT)
WHERE
  { ?s ?p ?o}
GROUP BY ?p

geometric mean

SELECT ?p (AGG<http://www.dotnetrdf.org/leviathan#geometric_mean>(?o) AS ?RESULT)
WHERE
  { ?s ?p ?o}
GROUP BY ?p

standard deviation

SELECT ?p (AGG<http://www.dotnetrdf.org/leviathan#standard_deviation>(?o) AS ?RESULT)
WHERE
  { ?s ?p ?o}
GROUP BY ?p

mode

SELECT ?p (AGG<http://www.dotnetrdf.org/leviathan#mode>(?o) AS ?RESULT)
WHERE
  { ?s ?p ?o}
GROUP BY ?p

median

SELECT ?p (AGG<http://www.dotnetrdf.org/leviathan#median>(?o) AS ?RESULT)
WHERE
  { ?s ?p ?o}
GROUP BY ?p

Custom aggregate functions can be freely mixed together or with standard SPARQL aggregate functions, such as:

SUM
MIN
MAX
SAMPLE
AVG
GROUPCONCAT

For example:

SELECT ?p 
    (AGG<http://www.dotnetrdf.org/leviathan#variance>(?o) AS ?RESULT1)

   (SUM (?o) AS ?RESULT2)
    (AGG<http://www.dotnetrdf.org/leviathan#median>(?o) AS ?RESULT3)

CAUTION: The DISTINCT flavors of custom aggregates are currently not supported.

Geometric Mean

Geometric mean is defined as the nth root of the product of n values. The product’s absolute value is used under the radical sign to avoid negative numbers. The result in this case will be zero. The product’s absolute value is used under the radical sign to avoid negative numbers. In other words, if the product happens to be negative, that value is negated to make it positive and then its root is retrieved.