Selecting values in attribute table which have alternate same values in two different columns

Question

I am looking for ways to select features in attribute table which have same values but in different column.

In the picture below as you can see "fid_2" has values which match "fid" but they are different features (shown by black arrow). Here I have manually selected the feature which match but I want to be able to select all three features like the one below automatically. Any ideas?

enter image description here

When "Select by expression" used
enter image description here

When I used the Field calculator
enter image description here

• Michael Stimson: Given the example it seems that an attribute join fid to fid_2 qgistutorials.com/en/docs/performing_table_joins.html might be helpful but be VERY careful with FIDs, they're transient. When joined a matched fid to fid_2 will have fully populated rows but an unmatched one will have empty joined columns. If there are more fields or more than one instance of a joined field the process becomes far more complicated.

Answer 1

Use the "Select by expression" from the Selection Toolbar

selection

with the following expression:

array_contains(
 string_to_array(
 concatenate_unique("fid_2")
 ),
 "fid")

Usually fields like "id" and "fid" are integers, in this case use a slightly difference expression:

array_contains(
 string_to_array(
 concatenate_unique(
 to_string("fid_2")
 )
 ),
 "fid")

It uses the following functions: array_contains(), string_to_array(), concatenate_unique(), and to_string().

P.S. I do not really understand whether you need to select "fid" in "fid_2", or vise versa.

• Anu Rai: It says the expression in invalid. Plus the picture shared is only a part of the selection, I have to do it for a lot of data.
• Taras: Please check the updates :)
• Anu Rai: I used the second expression which showed false for all. Did not do the selection I wanted. I wanted to select these in a bid to dissolve those features into one and did not want to do it manually. Any further help will be highly appreciated.
• Taras: That is weird. Can you please share a piece of your data or at least show how the above expression works/fails in your project for your layer?
• Babel: @Taras: proud to have an attribute bamed after you (OP's last screenshot)? ;-)