Return one image (and sitelinks) per item? - sparql

Question

I want to return a table where each row is a distinct toy item and there are columns for each toy's image and its sitelink count.
Q: Is there a better way to do this than what I finally did below? Why did I have to move labeling and sitelinks to the inner query?
Initially, I naively thought I could run the following query. But I discovered it created one row for each toy-image pair (I suppose it would return what I want if every image property had a priority-ranked image?). E.g., "gumball machine" (wd:Q1737075) has two rows, one for each of its two images.
SELECT ?item ?itemLabel ?image ?sitelinks WHERE {
?item wdt:P31 wd:Q11422; #toy, returns
wdt:P18 ?image;
wikibase:sitelinks ?sitelinks.
SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
}
ORDER BY DESC(?sitelinks)
Run it!
So then I ran the following query, which gives me what I want.
SELECT ?item ?itemLabel ?sitelinks ?image WHERE {
{
SELECT ?item ?itemLabel ?sitelinks (MAX(?_image) AS ?image) WHERE {
?item wdt:P31 wd:Q11422; #toys
wikibase:sitelinks ?sitelinks;
rdfs:label ?itemLabel;
wdt:P18 ?_image.
FILTER(LANG(?itemLabel)="en")
}
GROUP BY ?item ?itemLabel ?sitelinks
}
?item wdt:P18 ?image.
#rdfs:label ?itemLabel;
#wikibase:sitelinks ?sitelinks
#FILTER(LANG(?itemLabel)="en")
}
ORDER BY DESC(?sitelinks)
Run it!
But Is this right? Do I really need to nest queries in order to get one image per item?
Also, you can see from the commented lines that I initially tried running this with the labelling and sitelinks in the outer query. But that led to query timeouts. Why? Shouldn't that have been the more efficient construction, saving the labelling/sitelink work to the end where I have a smaller dataset after the inner query work?

Answer 1

In the second query, there is no need to wrap the inner portion with the outer portion. So the following works just fine
SELECT ?item ?itemLabel ?sitelinks (MAX(?_image) AS ?image)
WHERE {
?item wdt:P31 wd:Q11422;
wikibase:sitelinks ?sitelinks;
rdfs:label ?itemLabel;
wdt:P18 ?_image.
FILTER(LANG(?itemLabel)="en")
}
GROUP BY ?item ?itemLabel ?sitelinks
ORDER BY DESC(?sitelinks)
Run it!
And if you don't need a specific image or to maintain replicability, just use SAMPLE to avoid the extra operations with MAX:
SELECT ?item ?itemLabel ?sitelinks (SAMPLE(?_image) AS ?image)
WHERE {
?item wdt:P31 wd:Q11422;
wikibase:sitelinks ?sitelinks;
rdfs:label ?itemLabel;
wdt:P18 ?_image.
FILTER(LANG(?itemLabel)="en")
}
GROUP BY ?item ?itemLabel ?sitelinks
ORDER BY DESC(?sitelinks)
Run it!