First

Aggregation functions first

syntax

• first(a)
• first(a, relation)

definition

• first(a) results in a Parameter with the first of the non Null values of Attribute a.
• first(a, relation) results in an Attribute with the first of the non null values of attribute a, grouped by relation. The Domain unit of the resulting attribute is the Values unit of the relation.

applies to

• attribute a with Numeric, Point, string or bool Value type
• relation with value type of the group CanBeDomainUnit

conditions

1. The values unit of the resulting data item should match with regard to value type and Metric with the values unit of attribute a.
2. The domain of Argument a and relation must match.

performance

• Without relation: O(n) where n = number of elements in the domain of a. Scans until first non-null value is found.
• With relation: O(n) where n = number of elements in the domain of a. Tracks first non-null value per group during a single pass.

example

parameter<uint32> firstNrInh    := first(City/NrInhabitants);  result = 550
parameter<string> firstCityName := first(City/CityName);       result = 'Amsterdam'
parameter<bool>   firstIsCap    := first(City/IsCapital);      result = True

attribute<uint32> firstNrInhRegion    (Region) := first(City/NrInhabitants, City/Region_rel);
attribute<string> firstCityNameRegion (Region) := first(City/CityName,      City/Region_rel);
attribute<bool>   firstIsCapitalRegion (Region) := first(City/IsCapital,     City/Region_rel);

City/NrInhabitants	City/CityName	IsCapital	City/Region_rel
550	Amsterdam	True	0
525	Rotterdam	False	1
300	Utrecht	False	2
500	DenHaag	False	1
200	Eindhoven	False	3
175	Haarlem	False	null
null	null	False	3

domain City, nr of rows = 7

firstNrInhRegion	firstCityNameRegion	firstIsCapitalRegion
550	Amsterdam	True
525	Rotterdam	False
300	Utrecht	False
200	Eindhoven	False
null	null	False

domain Region, nr of rows = 5

see also

• Last