Multiple grouping
The principle of the multiple grouping is applied if within a single query the original data stock shall be grouped multiple times according to different criteria. Multiple grouping stands in opposition to a (onetime) grouping via several attributes. In the current example, the average age shall be calculated by gender as well as by occupational group with the following result:
<MedicalPersonnel> <MedicalPersonnel_according_Occupation> <OccupationalGroup> <Occupation>Doctor</Occupation> <Age>28</Age> </OccupationalGroup> <OccupationalGroup> <Occupation>Nurse</Occupation> <Age>53</Age> </OccupationalGroup> </MedicalPersonnel_according_Occupation> <MedicalPersonnel_according_Gender> <GenderGroup> <Gender>m</Gender> <Age>28</Age> </GenderGroup> <GenderGroup> <Gender>w</Gender> <Age>53</Age> </Geschlechtsgruppe> </MedicalPersonnel_according_Gender> </MedicalPersonnel>
The corresponding XQuery results from two consecutive FLWOR blocks and is as follows:
<MedicalPersonnel> <MedicalPersonnel_according_Occupation> { for $o in fn:distinct-values( for $i in fn:doc("...")//MedicalPersonnel/*/* return fn:name($i)) let $x := fn:doc("...")//*[fn:name(.) = $o]/Age return <OccupationalGroup> <Occupation>{ $o }</Occupation> <Age>{ fn:avg($x) }</Age> </OccupationalGroup> } </MedicalPersonnel_according_Occupation> <MedicalPersonnel_according_Gender> { for $g in fn:distinct-values(fn:doc("...")//Gender) let $x := fn:doc("...")//*[Gender=$g]/Age return <GenderGroup> <Gender>{ $g }</Gender> <Age>{ fn:avg($x) }</Age> </GenderGroup> } </MedicalPersonnel_according_Gender> </MedicalPersonnel>
In case two grouping combinations have equal grouping properties, the similarities can be used to the effect that only a single variable must be defined and can be used jointly.
Source: "XQuery – Grundlagen und fortgeschrittene Methoden", dpunkt-Verlag, Heidelberg (2004)
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