Splitting up a sequence - xquery

Question

I have just gotten started with xQuery as part of a course in databases. For the course we are using the mondial database found here (https://www.dbis.informatik.uni-goettingen.de/Mondial/). Basically the database contains information about the world, countries, lakes, rivers etc.
One assignment that we have is to take the area of all lakes that have an island and then group that by the continent. So far I have done this:
let $lakes :=
for $lakeList in doc("mondial.xml")/mondial/lake[#island]
for $country in doc("mondial.xml")/mondial/country
where $lakeList/located/#country = $country/#car_code
return <lake area = "{$lakeList/area}" country = "{$country/name}" continent = "{$country/encompassed/#continent}" continentPerc = "{$country/encompassed/#percentage}"></lake>
return ($lakes)
And this query generates the following result:
<lake area="40.2" country="Denmark" continent="europe" continentPerc="100"/>
<lake area="56" country="United Kingdom" continent="europe" continentPerc="100"/>
<lake area="71" country="United Kingdom" continent="europe" continentPerc="100"/>
<lake area="10" country="Russia" continent="europe asia" continentPerc="25 75"/>
<lake area="670" country="Japan" continent="asia" continentPerc="100"/>
<lake area="1.8" country="Philippines" continent="asia" continentPerc="100"/>
<lake area="234.2" country="Philippines" continent="asia" continentPerc="100"/>
<lake area="0.3" country="Philippines" continent="asia" continentPerc="100"/>
<lake area="911" country="Philippines" continent="asia" continentPerc="100"/>
<lake area="354.6" country="Philippines" continent="asia" continentPerc="100"/>
<lake area="1103" country="Indonesia" continent="asia australia" continentPerc="78 22"/>
<lake area="0.04" country="Indonesia" continent="asia australia" continentPerc="78 22"/>
<lake area="11.3" country="Indonesia" continent="asia australia" continentPerc="78 22"/>
<lake area="537.5" country="Canada" continent="america" continentPerc="100"/>
<lake area="5542" country="Canada" continent="america" continentPerc="100"/>
<lake area="104" country="Canada" continent="america" continentPerc="100"/>
<lake area="38.7" country="Canada" continent="america" continentPerc="100"/>
The problem with summing now seems to be that some attributes hold multiple values. So it becomes impossible to group by continent. Therefor I want to split up sequences that have multiple continent and continentPerc so that the area corrosponds to the respective continent.
Example of what I mean:
<lake area="1103" country="Indonesia" continent="asia australia" continentPerc="78 22"/>
Should become
<lake area="1103" country="Indonesia" continent="asia" continentPerc="78"/>
<lake area="1103" country="Indonesia" continent="australia" continentPerc="22"/>
Can this be achieved somehow or should I opt for another strategy?
I am very grateful for any help and pointers I can get, as I am very bad at Xquery. Thanks in advance.

Answer 1

I am not familiar with that database structure and I am not sure I have understood the problem (still wondering whether a lake can belong to Asia and Australia) but if you directly extract the continents and group the tuple stream on continent, as in
let $countries := doc('mondial.xml')/mondial/country
for $lake in doc("mondial.xml")/mondial/lake[#island]
for $continent in $countries[#car_code = tokenize($lake/#country, '\s+')]/encompassed/#continent
group by $continent
order by $continent
return <continent name="{$continent}">{$lake/<lake name="{#id}" area="{area}"/>}</continent>
then you get
<continent name="america">
<lake name="lake-Hazen" area="537.5"/>
<lake name="lake-Nettilling" area="5542"/>
<lake name="lake-Lake_Manitou" area="104"/>
<lake name="lake-Mindemoya" area="38.7"/>
</continent>
<continent name="asia">
<lake name="lake-KoltsevoyeLake" area="10"/>
<lake name="lake-Biwa" area="670"/>
<lake name="lake-Pinatubo" area="1.8"/>
<lake name="lake-Taal" area="234.2"/>
<lake name="lake-TaalCrater" area="0.3"/>
<lake name="lake-LagunaDeBay" area="911"/>
<lake name="lake-Lanao" area="354.6"/>
<lake name="lake-Toba" area="1103"/>
<lake name="lake-DanauKumbang" area="0.04"/>
<lake name="lake-SegaraAnak" area="11.3"/>
</continent>
<continent name="australia">
<lake name="lake-Toba" area="1103"/>
<lake name="lake-DanauKumbang" area="0.04"/>
<lake name="lake-SegaraAnak" area="11.3"/>
<lake name="lake-LakeTaupo" area="622"/>
<lake name="lake-Wanaka" area="192"/>
</continent>
<continent name="europe">
<lake name="lake-Arresoe" area="40.2"/>
<lake name="lake-LoughNeagh" area="392"/>
<lake name="lake-LochNess" area="56"/>
<lake name="lake-LochLomond" area="71"/>
<lake name="lake-KoltsevoyeLake" area="10"/>
</continent>
To try to incorporate the country and percentage data I have come up with
let $countries := doc('mondial.xml')/mondial/country
for $lake in doc("mondial.xml")/mondial/lake[#island]
for $lake-country in $countries[#car_code = $lake/located/#country]
for $continent in $lake-country/encompassed
group by $continent-name := $continent/#continent
order by $continent-name
return
<continent name="{$continent-name}">{
$lake!<lake name="{#id}"
area="{area}"
country="{let $l := . return $countries[#car_code = $l/located/#country and encompassed[#continent = $continent-name]]/name}"
percentage="{let $l := . return $countries[#car_code = $l/located/#country]/encompassed[#continent = $continent-name]/#percentage}"/>
}</continent>
I hope there are more elegant and compact ways to express that but it seems to do the job.