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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?

  • Change the double quote "mark" to singe quote 'mark'. Beside is it mark in K or marc in c as in your question. – ahmadhanb Dec 7 '17 at 10:57
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    @ahmadhanb - If you do that and there is at least one value of mark in the field, you will sum the entire field including those features which do not have mark :) – Joseph Dec 7 '17 at 11:06
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    @Joseph I see, I did not notice that. Thanks for your clarification. – ahmadhanb Dec 7 '17 at 11:10
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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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    It's better to update your answer to use 'marc' in 'c' since the asker modified his question. – ahmadhanb Dec 7 '17 at 11:36
  • In the expression builder I can see the right output, but when I try to get the value from my python script I always get None. Any ideas? Or should I post another question about this? – Blinxen Dec 7 '17 at 11:51
  • @Blinxen - I think you should post another question and include the script you used otherwise it is difficult to tell where the problem is :) – Joseph Dec 7 '17 at 11:53
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    I created a new question and posted it. – Blinxen Dec 7 '17 at 12:25

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