Add grouping variable to attribute table using QGIS

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I have a vector shapefile of river networks, where each reach in the networks has its own row in the attribute table. I want to add a grouping column to the attribute table indicating which network the reach belongs to, e.g.:

a two network example:

and my attribute table:

I would like an additional column in the attribute table called "network" which would be filled with 1, 2, ….n indicating which network the given reach belongs to (in the example above "network" would be 1 or 2 since there are only two networks), it doesn't matter which network is 1 and which is 2.

Best Answer

Apply a buffer around the line (network) layer and dissolve it. Convert the buffers from Multipart to single parts.
Create a unique id on the buffer layer with field calculator and $id. Than add this id to the line layer with Join attributes by location (see screenshot).

Buffer layer in orange and resulting line layer in black with id added to label the network lines:

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Autofilling columns in attribute table based on values of another cell in QGIS

Use the Field calculator with a CASE-statement. The expression looks like this, where "vegetation_type" is the name of the field containing the WC1, WC2 values:

case 
    when "vegetation_type" in ('WC1', 'WC2') then 'Wet Coniferous Forest Group'
end

Before the end statement, you can add as many similar when ... then statements as you like:

when "vegetation_type" in ('WC3', 'WC4') then 'Another Group'

You can also add an else statement that returns the output in all other cases, something like else 'other'. Add this after the last when statement, before end:

case 
    when "vegetation_type" in ('WC1', 'WC2') then 'Wet Coniferous Forest Group'
    when "vegetation_type" in ('WC3', 'WC4') then 'Another Group'
    else 'other'
end

QGIS show WKT from attribute table

If it's just for visualization you could use the geometry generator with the same expression:

geom_from_wkt("WKT")

To set it up you may duplicate your layer (so you will get one layer with the original geometry and the copy with the geometry from the WKT) or add a new symbol layer to display both geometry in original layer. Then in the Layer styling panel use the below setting (Symbol layer type : Geometry Generator ; Geometry type : Point/Multipoint):

Alternatively you can use the processing tool "Geometry by expression" (using again the same expression) but this will create a new layer so change on the original layer wont be reflected in this.

EDIT : If you have no geometry in the original layer the best solution is to create a virtual layer based on your table where your SQL query generate the geometry from the WKT column, this way you get a layer that will stay synchronized with the original data.

You may adapt this to get the layer (replace : "Field &" , ... , "Field n" by the name of the field you want to get and mysql_TABLE by the name of your table)

SQL query :

SELECT "Field &" , ... , "Field n" , st_geomfromtext( "WKT") as geometry FROM mysql_TABLE

Best Answer

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Autofilling columns in attribute table based on values of another cell in QGIS

QGIS show WKT from attribute table

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