QGIS attribution and geometry query

Question

I am trying to use select by expression to query lines that contain certain attributes and that there geometry touch. In other words are connected or neighbour each other.

I tried a query such as this. My attribution might not make sense but I just need the syntax correct.

"isDiscouraged" = 't' AND touches("roadclass" = 'ramp')

or

"Links that have this attribute" AND Touch "links that have this attribute"

So I want to find links that contain the attribute isdiscouraged = t that are connected to links with this other attribute roadclass = ramp. Is this possible?

My above query is invalid but lays out what I'm looking for. Any ideas how to tweek it to make it work.

I know its kind of a more spatial query combined with attribute query but can't get it to work!

Answer 1

But your features are all in the same layer? if yes you can't do a spatial query on the same layer. You need to duplicate the layer.

I suggest to do this:

layer_1->filter->"isdiscouraged" = 't'

copy_of_layer_1->filter-> "roadclass" = 'ramp'

Spatial query layer_1 touchs copy_of_layer_1

If you use postgresql and postgis you can do this with just one query

SELECT foo1.* FROM
(SELECT * FROM layer_1 WHERE isdiscouraged = 't') as foo1
(SELECT * FROM layer_1 WHERE roadclass = 'ramp') as foo2
WHERE st_touches(foo1.geom, foo2.geom);

I suggest to read what touches mean, beacause probably intersection is more usefull for you