XQuery get the highest value every day - xpath

Question

I'm practicing with XQuery and I've reached a point where I have to get the maximum values ​​of each day of an xml file.
My file deals with the maximum temperatures that a city has throughout the days. And it appears the same day several times but with different hours throughout the day. I will give you an example of how the data appear in the XML file:
<hour>
<date>2019-2-18</date>
<data_hour>12:00</data_hour>
<temperature>17</temperature>
</hour>
<hour>
<date>2019-2-18</date>
<data_hour>13:00</data_hour>
<temperature>18</temperature>
</hour>
<hour>
<date>2019-2-19</date>
<data_hour>14:00</data_hour>
<temperature>20</temperature>
</hour>
<hour>
<date>2019-2-19</date>
<data_hour>15:00</data_hour>
<temperature>19</temperature>
</hour>
The idea would be to take out the maximum hour of each day, any idea of ​​how to solve it?

Answer 1

If XQuery 3.0 is available, you could use the group by clause to group the hour elements together for each day, then apply the max function to select the highest temperature. In this code, I assume you've bound the hour elements to a variable $hours:
for $hour in $hours
group by $date := $hour/date
return
$date || ": " || max($hour/temperature)
This returns two items given your source data:
"2019-2-19: 20",
"2019-2-18: 18"
I will leave it as an exercise for you to sort the results by date. In the process, you will likely hit the limits of the current formatting of your dates, so you will need to convert the dates to values that conform to the xs:date data type, which is reliably sortable.
If you are limited to XQuery 1.0, then see XQuery 3.0 equivalent group by in xquery 1.0 version.

Answer 2

Try this, add an additional parent tag like I've added
let $body := doc("1.xml")
for $i in distinct-values($body//hour/date)
return <Day value = "{data($i)}"> {
let $hours := (for $row in $body//hour
for $j in $row/date where xs:string(data($j)) = xs:string(data($i))
return fn:substring($row/data_hour, 1, 2))
return fn:max($hours)
}
</Day>
Tell me if minutes are also considered.