How to count nodes for child item having same date - xquery

Question

I have Xml structures coming like this below. I will have to count the installations on the same date and list them. So I will have to find count by InstallDate and return in XQuery to be used in Marklogic query.
<PcDetail>
<Installations>
<Installation>
<Type>pc</Type>
<purchased>Y</purchased>
<InstallDate>2020-10-01T00:00:00:000Z</InstallDate>
</Installation>
<Installation>
<Type>pc</Type>
<purchased>Y</purchased>
<InstallDate>2020-10-01T00:00:00:000Z</InstallDate>
</Installation>
<Installation>
<Type>pc</Type>
<purchased>Y</purchased>
<InstallDate>2020-10-02T00:00:00:000Z</InstallDate>
</Installation>
<Installation>
<Type>pc</Type>
<purchased>Y</purchased>
<InstallDate>2020-10-02T00:00:00:000Z</InstallDate>
</Installation>
<Installation>
<Type>pc</Type>
<purchased>Y</purchased>
<InstallDate>2020-10-02T00:00:00:000Z</InstallDate>
</Installation>
</Installations>

Answer 1

If you wish to group, count and list the Installation by date in MarkLogic server, see below MarkLogic XQuery dialect:
for $d in distinct-values(doc()//InstallDate)
let $i := doc()//Installation[InstallDate = $d]
order by $d
return <Installations Date="{$d}" Count="{count($i)}">
{
for $install in $i
return $install
}</Installations>
In a transaction-driven environment, you should utilise MarkLogic performant search engine:
for $vt in cts:value-tuples(
(cts:element-reference(xs:QName( 'InstallDate'))
),("item-frequency")
)
return "Installation on " || $vt[1] || " total count: " || cts:frequency($vt)

Answer 2

It's pretty straightforward even in XQuery 1.0
let $installations := $xml/PcDetail/Installations/Installation
for $date in fn:distinct-values($installations/InstallDate)
let $installationsOnDate := $installations[InstallDate = $date]
let $c := count($installationsOnDate)
return $date || ':' || $c || ':' || string-join($installationsOnDate/type,', ')