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How can I dissolve a polygon layer in QGIS based on a single attribute but keep all (or some) other attributes at the same time? This is how I want to retain fields:

There are at least two fields I want to retain the information from in the dissolved shapefile, one is a string, the other one is numerical.

I would like to dissolve the shapefile based on the field "Class", and I want to retain the information of columns "Object_ID" and "Signal_species"

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It is now more than one year from your post... From QGIS 2.16, new Aggregates functions became available. One of the new expression is concatenate().

Your Sample

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concatenate()

Open the attribute table and create a new text field (with enough string length), with expression:

# Object_ID  -> O_IDs 
concatenate(  "Object_ID" , group_by:= "Class", concatenator:= ', ' )

# Signal_Sp  -> Signals
concatenate( to_string("Signal_Sp"), group_by:= "Class", concatenator:= ', ' )

Please note concatenate() takes only texts (strings), so I had to apply to_string() to "Signal_Sp" which was integer type...

After concatenate() your attribute table will become like below:

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Now we can dissolve the polygon keeping original attribute data.

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I will need to delete old fields (Object_ID and Signal_Sp) afterwards.

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