I have a complex workflow prepared in the processing modeler, but I'm missing one step to link all the model.

I need to dissolve a polyline vector in a single line vector that has as attribute the SUM of a specified attribute of the polyline vector. I can do it via plugin (dissolve with stats) but just outside the modeler, I tried to find a way to link the "dissolve" tool and the "basic statistics for numeric field" tool but I still can't figure out how to put the html output inside the vector layer.

I saw some comments about an sql query that looks easy, but I don't know if I can use it outside spatialite or sql, inside the modeller.

I tried the command SELECT SUM(column_name) FROM table_name;

but as out put i got GDAL command output: FAILURE: Unable to open datasource

Is it because the layer has to be inside an sql database to run this kind of query?


3 Answers 3


You can use gdal/ogr tool inside your modelbuilder :

enter image description here

hope that helps


Ujaval gandhi from qgistutorials prepared a python script to be run inside the modeler that solves the problem, here is the tutorial that Ujaval made for everyone. http://www.qgistutorials.com/en/docs/processing_python_scripts.html

  • 2
    Yes, brilliant tutorials! You should edit your answer to include the code you linked. Just in case the link is broken, the code will still be available for users =)
    – Joseph
    Commented Aug 11, 2015 at 10:30

You can also use the ogr2ogr based tool "Dissolve polygons" and click on "compute min/max/sum/mean for the following attribute". It executes this query:

ogr2ogr "[temporary file]" folder/file.shp file -dialect sqlite -sql "SELECT ST_Union(geometry),Column FROM 'file' GROUP BY Column"

Available in QGIS >= 2.10

enter image description here

  • This tool works only for polygons. the question is about polylines
    – Mattia
    Commented Sep 21, 2015 at 7:56
  • 2
    Hi, I'm the author of "dissolve polygons" inside the OGR category in the QGIS Processing toolbox. If you think that a "dissolve lines" tool is handy then please open a feature request on the QGIS bug tracker and assign it to me. Cheers.
    – gioman
    Commented Sep 21, 2015 at 8:06
  • @GiovanniManghi done: hub.qgis.org/issues/13386
    – Mattia
    Commented Sep 21, 2015 at 9:12

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