[Qgis-user] Using aggregates functions to count intersected features from another layer

Wed Oct 19 09:04:14 PDT 2016

Hi All,

Harrissou, have a look at [1] for a very similar question and one possible
solution using the Function Editor (of course, instead of sum you would use
count).

Do you other devs find that approach useful? Any hint for keeping the index
up-to-date?

Is there any interest in having this kind of functions available for QGIS
expressions? I could help with that.

Regards,

Germán
--
[1] http://gis.stackexchange.com/a/212762/4972

2016-10-19 7:42 GMT-05:00 Neumann, Andreas <a.neumann at carto.net>:

> Good idea to use a virtual layer for that! It is probably also more
> efficient than a QGIS expression.
>
> Just to understand the virtual layers correctly:
>
> when is the query behind the virtual layer executed?
>
> - at every redraw?
>
> - once at creation?
>
> - at project reload?
>
> Aren't the virtual layers automatically updated, at least when the project
> reloads?
>
> Are the results cached or not?
>
> Would also make sense to add a "update" option for a virtual layer, if
> that doesn't exist yet.
>
> Greetings,
>
> Andreas
>
>
>
> On 2016-10-19 14:32, Nathan Woodrow wrote:
>
> Maybe using a virtual layer query via DB Manager will work.  You have full
> SQL join support. Speed isn't always the best but gives you the full power
> of SQL which is the correct way to do this IMO.
>
> E.g SELECT LayerA.id, COUNT(*) FROM layerA
> JOIN layerB ON ST_WithIn(layerA.geometry, layerB.geometry)
> GROUP BY layerA.type
>
> Although you can't do an update but you can make a new layer with the new
> counts.
>
> - Nathan
>
> On Wed, Oct 19, 2016 at 10:22 PM, Neumann, Andreas <a.neumann at carto.net>
> wrote:
>
>> Well yes - but that is not a "live" point in polygon. It create separate
>> layers which need to be kept up-to-date.
>>
>> Harrissou was after a "live" point in polygon, which is automatically
>> kept up-to-date through a QGIS expression.
>>
>> Maybe Nyall knows how to write the correct aggregate expression for that?
>>
>> Greetings,
>>
>> Andreas
>>
>> On 2016-10-19 14:11, Nicolas Cadieux wrote:
>>
>>
>> Try this.
>>
>> http://www.qgistutorials.com/fr/docs/points_in_polygon.html
>>
>> Nicolas
>>
>> Le 19 oct. 2016 à 06:37, DelazJ [via OSGeo.org] <[hidden email]> a
>> écrit :
>>
>> Hi,
>>
>> I have a polygon layer and a point one.
>> I'd like to calculate for each polygon feature, the number of points that
>> it covers. I thought the aggregates functions could help me find an easier
>> and straight expression to perform this (without creating intermediate
>> layers) but I fail to find any correct syntax.
>>
>> I tried aggregate ('mypoint', 'count', "id", intersects ($geometry,
>> geometry($currentfeature)))
>>
>>  And many variants of the fourth option but It always return the total of
>> points or an error. Actually this option is a filter and I wonder if i'm
>> not missing a "group by" option in the aggregate function (which seems to
>> be the appropriate one in my case). Is it possible?
>> The work around I found is to populate an ad'hoc field in the point layer
>> with overlapping polygon id, thanks to the SpatialJoin plugin, then I
>> create a one-to-many relation in the project between the two layers in
>> other to use relation_aggregate function.
>> But I expected something more direct.
>>
>> Any hint, please?
>>
>> Thanks,
>> Harrissou
>>
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