On 09.10.2017 10:22, George News wrote:
> Hi all,
>
> Here it goes. The MWE is below.
>
> As you can see when I execute Q_without.rq I get an empty array in bindings.
> However with Qmax_without.rq I get {} (empty object) in bindings.
>
> What I'm saying is that if I'm getting nothing when listing the matching
> entities, I don't know why I get an empty object when listing the matching
> entities with an aggregate operation like MAX.
Well, I guess Andy already gave you the answer in the beginning of the
thread:
A query with an aggregate always returns a row by convention. But
compared to the aggregate function COUNT which simply can return 0 then,
MAX can't return any concrete value because the MAX of nothing is not
specified.
> My opinion is that it should be an empty resultset as there is nothing
> matching. That is, I cannot the max of nothing.
>
> Hope now it is clear, and sorry for this long thread.
>
> Regards,
> Jorge
>
>
>
> $ cat dataset.ttl
> <http://example.org/#a> <http://myschema/name> "Jorge" .
> <http://example.org/#a> <http://myschema/age>
> "30"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http://example.org/#a> <http://myschema/age>
> "20"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http://example.org/#b> <http://myschema/name> "Jorge" .
> <http://example.org/#b> <http://myschema/age>
> "5"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http;//example.org/#b> <http://myschema/age>
> "80"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http://example.org/#c> <http://myschema/name> "Jorge" .
> <http://example.org/#c> <http://myschema/age>
> "8"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http://example.org/#c> <http://myschema/age>
> "32"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http://example.org/#d> <http://myschema/name> "Jo" .
> <http://example.org/#d> <http://myschema/age>
> "3"^^<http://www.w3.org/2001/XMLSchema#integer> .
> <http://example.org/#d> <http://myschema/age>
> "2"^^<http://www.w3.org/2001/XMLSchema#integer> .
>
> $ cat Q_with.rq
> SELECT ?entity ?age
> WHERE {
> ?entity <http://myschema/name> "Jo" .
> ?entity <http://myschema/age> ?age .
> }
>
> $ /opt/apache-jena/bin/sparql --query Q_with.rq --data dataset.ttl --results
> json
> {
> "head": {
> "vars": [ "entity" , "age" ]
> } ,
> "results": {
> "bindings": [
> {
> "entity": { "type": "uri" , "value": "http://example.org/#d"; } ,
> "age": { "type": "literal" , "datatype":
> "http://www.w3.org/2001/XMLSchema#integer"; , "value": "3" }
> } ,
> {
> "entity": { "type": "uri" , "value": "http://example.org/#d"; } ,
> "age": { "type": "literal" , "datatype":
> "http://www.w3.org/2001/XMLSchema#integer"; , "value": "2" }
> }
> ]
> }
> }
>
> $ cat Q_without.rq
> SELECT ?entity ?age
> WHERE {
> ?entity <http://myschema/name> "J" .
> ?entity <http://myschema/age> ?age .
> }
>
> $ /opt/apache-jena/bin/sparql --query Q_without.rq --data dataset.ttl
> --results json
> {
> "head": {
> "vars": [ "entity" , "age" ]
> } ,
> "results": {
> "bindings": [
>
> ]
> }
> }
>
> $ cat Qmax_with.rq
> SELECT ?entity (MAX(?age) AS ?value)
> WHERE {
> ?entity <http://myschema/name> "Jo" .
> ?entity <http://myschema/age> ?age .
> } group by ?entity
>
> $ /opt/apache-jena/bin/sparql --query Qmax_with.rq --data dataset.ttl
> --results json
> {
> "head": {
> "vars": [ "entity" , "value" ]
> } ,
> "results": {
> "bindings": [
> {
> "entity": { "type": "uri" , "value": "http://example.org/#d"; } ,
> "value": { "type": "literal" , "datatype":
> "http://www.w3.org/2001/XMLSchema#integer"; , "value": "3" }
> }
> ]
> }
> }
>
> $ cat Qmax_without.rq
> SELECT ?entity (MAX(?age) AS ?value)
> WHERE {
> ?entity <http://myschema/name> "J" .
> ?entity <http://myschema/age> ?age .
> } group by ?entity
>
> $ /opt/apache-jena/bin/sparql --query Qmax_without.rq --data dataset.ttl
> --results json
> {
> "head": {
> "vars": [ "entity" , "value" ]
> } ,
> "results": {
> "bindings": [
> {
>
> }
> ]
> }
> }
>
>
>
>
>
>
> On 2017-10-09 09:25, George News wrote:
>> On 2017-10-08 11:18, Andy Seaborne wrote:
>>> Please - a complete, verifiable, minimal example.
>> I will try to compile one simple :(
>>
>>> Complete query, small amount of data.
>>>
>>> Snippets remove details that may matter.
>>>
>>> With a complete, verifiable, minimal example we can agree on what the
>>> question is.
>>>
>>> Indeed, it is not clear your output is even from Jena. The JSON
>>> formatting is not as Jena emits it.
>> I just used the Jena one. Only remove newlines in vars to make it shorter.
>>
>>> Andy
>>>
>>> On 08/10/17 01:37, George News wrote:
>>>> Hi,
>>>>
>>>> Forget the last one. I've just realized again I included a mistake....
>>>> this is the good one (I hope ;))
>>>>
>>>> # Case 1)
>>>> select ?id ?value ?latitude ?longitude
>>>> where {
>>>> ......
>>>> }
>>>> ----------
>>>> {
>>>> "head": {
>>>> "vars": [
>>>> "id", "value", "latitude", "longitude"
>>>> ]
>>>> },
>>>> "results": {
>>>> "bindings": [
>>>> ]
>>>> }
>>>> }
>>>>
>>>> # Case 2)
>>>> select ?id (MAX(?ti) as ?time) ?value ?latitude ?longitude
>>>> where {
>>>> ......
>>>> }
>>>> ----------
>>>> {
>>>> "head": {
>>>> "vars": [
>>>> "id", "time", "value", "latitude", "longitude"
>>>> ]
>>>> },
>>>> "results": {
>>>> "bindings": [
>>>> {}
>>>> ]
>>>> }
>>>> }
>>>>
>>>> Now you can see the difference I was noticing. In the first case
>>>> bindings is an empty array (resultset.hasNext() -> false) and the second
>>>> is an array with an empty object (resultset.hasNext() -> true).
>>>>
>>>> Why is this behaviour? Hope you now understand the issue which in my
>>>> opinion is a kind of a bug.
>>>>
>>>> Regards,
>>>> Jorge
>>>>
>>>> On 2017-10-08 00:15, George News wrote:
>>>>> Hi Andy,
>>>>>
>>>>> Now I understand the misunderstanding between you and me. The responses
>>>>> I included in my original mail where wrong :( Please accept my
>>>>> apologizes.
>>>>>
>>>>> These are the right query/responses:
>>>>>
>>>>> # Case 1)
>>>>> select ?id (MAX(?ti) as ?time) ?value ?latitude ?longitude
>>>>> where {
>>>>> ......
>>>>> }----------
>>>>> {
>>>>> "head": {
>>>>> "vars": [
>>>>> "id", "time", "value", "latitude", "longitude"
>>>>> ]
>>>>> },
>>>>> "results": {
>>>>> "bindings": [
>>>>> {}
>>>>> ]
>>>>> }
>>>>> }
>>>>>
>>>>> # Case 2)
>>>>> select ?id (MAX(?ti) as ?time) ?value ?latitude ?longitude
>>>>> where {
>>>>> ......
>>>>> }----------
>>>>> {
>>>>> "head": {
>>>>> "vars": [
>>>>> "id", "value", "latitude", "longitude"
>>>>> ]
>>>>> },
>>>>> "results": {
>>>>> "bindings": [
>>>>> ]
>>>>> }
>>>>> }
>>>>>
>>>>> Now you can see the difference I was noticing. In the first case it is
>>>>> an empty array (resultset.hasNext() -> false) and the second is an array
>>>>> with an empty object (resultset.hasNext() -> true).
>>>>>
>>>>> Why is this behaviour? Hope you now understand the issue which in my
>>>>> opinion is a kind of a bug.
>>>>>
>>>>> Regards,
>>>>> Jorge
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On 2017-10-06 16:11, Andy Seaborne wrote:
>>>>>>
>>>>>> On 06/10/17 12:26, George News wrote:
>>>>>>> On 2017-10-06 11:25, Andy Seaborne wrote:
>>>>>>>> The two result sets you show both have one row, with bindings. That's
>>>>>>>> consistent with aggregation of nothing (no groups, or if no GROUP
>>>>>>>> BY, no
>>>>>>>> results from the WHERE pattern.
>>>>>>> I don't see it the same way. The first one (without max) is an empty
>>>>>>> array, while the second (with max) has an array with one object
>>>>>>> (empty).
>>>>>> "results": {
>>>>>> "bindings": [
>>>>>> {}
>>>>>> ]
>>>>>> }
>>>>>>
>>>>>> both times.
>>>>>>
>>>>>> An array of rows, a row is {} i.e. no keys, no variables.
>>>>>>
>>>>>> But the query isn't legal so I don't know what is actually happening.
>>>>>>
>>>>>>>> MAX() of nothing is unbound but for any aggregation, there always is
>>>>>>>> a row/
>>>>>>>>
>>>>>>>> c.f. COUNT(*) is 0 when there are no solution.
>>>>>>>>
>>>>>>>> It's just MAX(...) can't return a "there isn't anything value"
>>>>>>>>
>>>>>>>> Andy
>>>>>>>>
>>>>>>> I see your point as this gives a wrong idea on the result set as it
>>>>>>> really is empty. If I dont get any time I cannot calculate the max of
>>>>>>> nothing. In principle this is what Jena is returning as the object is
>>>>>>> empty, but there should be a way to not get this empty object
>>>>>>> within the
>>>>>>> array of bindings.
>>>>>>>
>>>>>>> Is there anyway I can check the resultset pointer to get the next()
>>>>>>> value without moving the pointer? I need to know in advance to
>>>>>>> retrieve
>>>>>>> all the results if there are or aren't any.
>>>>>>>
>>>>>>>
>>>>>>>> On 06/10/17 10:15, George News wrote:
>>>>>>>>> Hi all,
>>>>>>>>>
>>>>>>>>> I am executing a SPARQL with MAX aggregate function and I'm facing a
>>>>>>>>> strange behaviour, or at least I think it is.
>>>>>>>>>
>>>>>>>>> The snipset of the select variables is the following:
>>>>>>>>>
>>>>>>>>> select ?id (MAX(?ti) as ?time) ?value ?latitude ?longitude
>>>>>>>>> where {
>>>>>>>>> ......
>>>>>>>>> }
>>>>>>>>> If I launch the SPARQL query and there are results matching there
>>>>>>>>> is no
>>>>>>>>> problem and I get the expected answer.
>>>>>>>>>
>>>>>>>>> However if I launch the same query over another database and there
>>>>>>>>> should be no match I get the following:
>>>>>>>>>
>>>>>>>>> {
>>>>>>>>> "head": {
>>>>>>>>> "vars": [
>>>>>>>>> "id", "time", "value", "latitude", "longitude"
>>>>>>>>> ]
>>>>>>>>> },
>>>>>>>>> "results": {
>>>>>>>>> "bindings": [
>>>>>>>>> {}
>>>>>>>>> ]
>>>>>>>>> }
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> As you can see, although the resultset seems to be empty it is
>>>>>>>>> not. It
>>>>>>>>> is returning one empty object. Actually by checking
>>>>>>>>> resultset.hasNext()
>>>>>>>>> within the code it returns true.
>>>>>>>>>
>>>>>>>>> If I remove the MAX function from the variables everything is ok,
>>>>>>>>> and no
>>>>>>>>> empty object shows up.
>>>>>>>>>
>>>>>>>>> select ?id ?value ?latitude ?longitude
>>>>>>>>> where {
>>>>>>>>> ......
>>>>>>>>> }
>>>>>>>>> ----------
>>>>>>>>> {
>>>>>>>>> "head": {
>>>>>>>>> "vars": [
>>>>>>>>> "id", "value", "latitude", "longitude"
>>>>>>>>> ]
>>>>>>>>> },
>>>>>>>>> "results": {
>>>>>>>>> "bindings": [
>>>>>>>>> {}
>>>>>>>>> ]
>>>>>>>>> }
>>>>>>>>> }
>>>>>>>>>
>>>>>>>>> Why is happening that? Is this the expected behaviour? I guess it
>>>>>>>>> shouldn't. When you use COUNT funtion it returns 0, but MIN/MAX/etc
>>>>>>>>> arer
>>>>>>>>> different functions and if there is no result nothing should appear.
>>>>>>>>>
>>>>>>>>> Any help/tip is more than welcome.
>>>>>>>>>
>>>>>>>>> Regards,
>>>>>>>>> Jorge
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
