Somehow the below times after once OPTIONAL {} is added:
```
SELECT
?item ?itemLabel
(GROUP_CONCAT(DISTINCT ?instanceLabel; separator=", ") as ?i)
(SAMPLE(?metricalPattern) as ?mp)
WHERE
{
?item wdt:P31/wdt:P279* wd:Q1651051 .
?item wdt:P31 ?instance .
OPTIONAL { ?item wdt:P2552 ?metricalPattern }.
SERVICE wikibase:label { bd:serviceParam wikibase:language "it,en" . ?instance rdfs:label ?instanceLabel . ?item rdfs:label ?itemLabel }
}
GROUP BY ?item ?itemLabel ?metricalPattern
```
[[ https://query.wikidata.org/#SELECT%20%09%0A%09%3Fitem%20%3FitemLabel%0A%09%28GROUP_CONCAT%28DISTINCT%20%3FinstanceLabel%3B%20separator%3D%22%2C%20%22%29%20as%20%3Fi%29%0A%09%28SAMPLE%28%3FmetricalPattern%29%20as%20%3Fmp%29%0AWHERE%20%0A%7B%0A%20%20%09%3Fitem%20wdt%3AP31%2Fwdt%3AP279%2a%20wd%3AQ1651051%20.%0A%09%3Fitem%20wdt%3AP31%20%3Finstance%20.%20%0A%09OPTIONAL%20%7B%20%3Fitem%20wdt%3AP2552%20%3FmetricalPattern%20%7D.%0A%09SERVICE%20wikibase%3Alabel%20%7B%20bd%3AserviceParam%20wikibase%3Alanguage%20%22it%2Cen%22%20.%20%3Finstance%20rdfs%3Alabel%20%3FinstanceLabel%20.%20%3Fitem%20rdfs%3Alabel%20%3FitemLabel%20%7D%0A%7D%0AGROUP%20BY%20%3Fitem%20%3FitemLabel%20%3FmetricalPattern | try it ]]
More complex queries work (see [[ https://www.wikidata.org/wiki/Wikidata_talk:SPARQL_query_service/queries#Optional_and_Group_by | here ]]). The main difference seems to be the concat on labels.
How can this be improved?