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I need to merge adjacent lines in QGIS 3.8, but am having trouble coming up with a way to identify and then merge them in a large data set. (3.8 is as far as my Mac can support)

I tried to convert my lines to polygons and use the "Join multiple lines" plugin. With both of these it seems like I'll need to select everything manually (not a feasible option) first or else it gets messy.

Ex. I want to locate lines like on the right here with touching endpoints, and merge them into one like on the left.

enter image description here

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One solution would be to union and merge touching lines in a self join SQL, then run a multipart to singlepart geometry with the QGIS geometry tools.

Make a virtual layer - where mylines is your layername - like

select st_linemerge(st_union(m1.geometry)) as geometry
from mylines m1
inner join mylines m2 on st_touches(m1.geometry, m2.geometry)

Then on the virtual layer run Vector > Geometry tools > Multipart to Singleparts

Anyone knows how to include the Multipart to Singleparts in SQL - I am happy to hear ..

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  • You can use recursive functions to try and emulate ST_Dump() (the equivalent function in PostGIS). See examples in these answers: gis.stackexchange.com/a/341570/98784 and Chainage points using virtual layer
    – she_weeds
    Apr 14, 2020 at 2:25
  • Here is an example query: WITH RECURSIVE rng(x) AS( SELECT 1 UNION ALL SELECT (x+1) FROM rng LIMIT ( SELECT cast(st_numgeometries(geometry) AS int) FROM line_collected)), line_collected AS (SELECT st_linemerge(st_union(m1.geometry)) as geometry, m1.id FROM mylines m1 INNER JOIN mylines m2 ON st_touches(m1.geometry, m2.geometry)) SELECT geometry_n(lc.geometry, rng.x) AS geometry, rng.x AS part_num FROM rng, line_collected lc;
    – she_weeds
    Apr 14, 2020 at 2:28

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