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So I want to combine all the polygons that share two different attribute values and don't really know where to start.

enter image description here

The selection shows you my problem, they both have the same classification (890) and belong to the same block (third column). I want these (and all other similar occurences) to be considered a single multipart polygon. How?

The regular singlepart to multipart tool won't do me any good since I will loose either classification field or the block id field.

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I would use the Dissolve geoprocessing tool:

Vector > Geoprocessing Tools > Dissolve...

Then, specify the column on which to merge polygons (in your case: JG2).

I did the test and it works good, you do preserve your 3 columns.

If you want to merge polygons based on 2 columns only, then concatenate the content of the two columns in a new one (string type) and use the Dissolve tool on this new column. This can be done easily with the Field Calculator using the following expression:

"JG2" || '_' || "Blockid"
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  • The problem with your first answer is the same as with doing single part to multi part. Yes I do get to keep all the columns, but the values in those columns get thrown away. But the string concatenation thing did just what I tried to do! Thanks!
    – Iamlukesky
    Nov 17, 2015 at 13:45
  • I did not get values thrown away problem with Dissolve tool... (?)
    – wiltomap
    Nov 17, 2015 at 13:55

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