get grouped results with sparql query - sparql

Question

I still feel like a SPARQL newbie, so I may be way off base about what SPARQL GROUP BY does, but here's my questions.
Suppose I wanted to request all resources in graph database called Categories, and I wanted to get all the items associated with these categories, along with the names of the items and their price.
Right now my SPARQL queries are giving me back something like the following table:
**Categories Item ItemName ItemPrice**
Tools HammerID Hammer $12
Tools SawID Saw $13
Tools WrenchID Wrench $10
Food AppleID Apple $5
Food CornID Corn $1
I wanted to use GROUP BY to group the items under a single category, so that when I start processing it, I can look through each unique category and then display the items that belong in that category.
Right now if I loop through the above results, I will be iterating over 5 entries instead of 2.
The other way I can describe the results I want are by imaging what the corresponding json data would look like. I want something like:
[
tools: [
{id: hammerId
title: hammer
price: $12},
{id: sawId
title: saw
price: $13},
{id: wrenchId
title: wrench
price: $10}
],
food: [
{id: appleId
title: apple
price: $5},
{id: cornId
title: corn
price: $1}
]
]
With the results, like this I can directly loop over the top level items, and then display the results for each.
Can I use GROUP BY to tell SPARQL to give me results like this?

Answer 1

No, you can't. A SPARQL SELECT query-result is defined as a sequence of solutions, with each solution being a set of variable-value pairs (with a value being defined as an IRI, BNode, or literal value). Basically it's a simple table. There is no provision for 'nested' solutions like you'd need for your JSON-like structure.
However the difference is purely syntactic. If you group, you know the result will deliver all solutions belonging to the same group together (one after the other) - so in processing the result you can simply treat the grouped variable as a marker. And of course if you really want, you can easily rewrite the query result into this kind of syntactic structure yourself - it's just a different way of writing down the exact same information, after all.