[GIS] Expression string builder QGIS

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I created a virtual field in my layer where I want a specific value to be calculated. In my attribute table I have several columns. The expression I am trying to create is the following. I want to iterate through all features and add all values of the column age who have a specific value, let's say the name of the features should be marc. In my first attempt I created this:

CASE WHEN  "name"  = "marc" THEN  sum(  "age" ) END

But it kinda doesn't work and I don't know how to iterate though all features in the layer and only choose those features with the name marc. Any advice?

Best Answer

You could use something like the following:

sum( "age", "name", "name" = 'marc' )

Where:

"age" is the field used to calculate the sum;
"name" is the field used to group the different names together;
"name" = 'marc' is the expression used to filter all names in the name field which equal marc.

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[GIS] Updating attribute field with random string value

You may create your own custom python function through field calculator function editor to create random strings to populate a field:

Click on the function editor tab within field cal
Create new file and copy in code below

Load the function

from qgis.core import *
from qgis.gui import *
import random, string

@qgsfunction(args='auto', group='Custom')
def myFun(value1, feature, parent):
    return randomword(10)

def randomword(length):
    return ''.join(random.choice(string.lowercase) for i in range(length))

Make a call under expression tab like and click Ok button to run:

myFun("fieldNameToBeUpdated")

QGIS – Grouping by Expression in Attributes Form Completion

Try using this expression (QGIS 3.22) in the 'Filter expression' part of your Value Relation widget -

$id=array_first(array_agg($id,"culture"))

It groups the feature id value from the other layer by the desired column, then just grabs the feature with the first id value from that list. The list is therefore filtered to just the first instance of each "culture" value. It's similar to the 'first value' aggregate that is present in several languages/tools, I'm not sure if Q 3.24 has introduced this as its own function.

I'm not sure if it will be very efficient if the source table is quite large though.

Example:

If you know your source layer will be updated regularly/dynamically, $id (QGIS interpreted feature id) can be a little unpredictable (esp with unsaved features), so instead of using $id in the above expression, you could also use a column from the source layer that you know will be unique and non-null every time a new feature is created (e.g. "uuid" or "id")

Best Answer

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[GIS] Updating attribute field with random string value

QGIS – Grouping by Expression in Attributes Form Completion

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