Covariance

Aggregation functions co-variance

syntax

• cov(a, b)
• cov(a, b, relation)

definition

• cov(a, b) results in a parameter with the co-variance of attributes a and b.
• cov(a, b, relation) results in an attribute with the co-variance of attributes a and b, grouped by relation. The domain unit of the resulting attribute is the values unit of the relation.

The value type of the resulting data item is float32 or float64.

applies to

• attribute a and b with Numeric value type
• relation with value type of the group CanBeDomainUnit

conditions

1. The value type of the arguments a and b must match.
2. The domain of arguments a, b and relation must match.

example

1. parameter<float32> covNrInhabitantsTemp                := 
   cov(float32(City/NrInhabitants), City/avgDailyTemperature); result = -156.25
2. attribute<float32> covNrInhabitantsTempRegion (Region) :=
   cov(
       float32(City/NrInhabitants)
     , City/avgDailyTemperature
     , City/RegionNr
  );

City/NrInhabitants	City/avgDailyTemperature	City/Region_rel
550	12	0
525	11	1
300	null	2
500	11	1
200	14	3
175	null	null
null	14	3

domain City, nr of rows = 7

covNrInhabitantsTempRegion
0
0
null
0
null

domain Region, nr of rows = 5