Counting layer segments in QGIS Field Calculator

Question

I have a segmented line (road) dataset and I want to get the number of times each road is segmented.

In SQL it's just

SELECT RoadMntnc, count(RoadMntnc) AS count
FROM clip_DSC_Roads
GROUP BY RoadMntnc

How do I do the same in field calculator?

I assume it would be something like aggregate(count("RoadMntnc"), group_by="RoadMntnc") but this doesn't work.

eurojam · Accepted Answer · 2019-09-05 06:12:54Z

5

EDIT: or to your changed request it should be:

array_length( array_agg( "RoadMntnc", "RoadMntnc"))

You can use the following expression to count the segments of a line or polygon:

num_geometries(segments_to_lines($geometry))

answered Sep 5, 2019 at 4:02

eurojam

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Thanks - this works but I didn't explain what I meant by segments properly. I don't mean the number of vertices on the line (each time it bends). What I meant is that a single road is made up of multiple lines with the same road name. I want to get the number of lines that make up a road. The SQL in the question does this...I want to do the same in field calculator. It should be some form of the aggregate/group_by expression but I can't figure it out.
– GeorgeC
Commented Sep 5, 2019 at 5:43
this should be: array_length( array_agg( "RoadMntnc", "RoadMntnc"))
– eurojam
Commented Sep 5, 2019 at 6:09
-brilliant. That works well. Good value for money on this question where you have given 2 great answers that will both be helpful. :-)
– GeorgeC
Commented Sep 5, 2019 at 6:13

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