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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