Dissolving polygon with enclave does not work

Question

i tried to dissolve two polygons(brown and the yellow marked polygons) of one layer that own some en-/exclaves in QGIS (first union both, then dissolve the union-polygon).

But dissolving dose not work :(. I don't get an error but it dissolved only one polygon an the other one is not included. There are also other polygons without enclaves and it works perfect.

The data I used is opendata (http://www.geodatenzentrum.de/geodaten/gdz_rahmen.gdz_div?gdz_spr=deu&gdz_akt_zeile=5&gdz_anz_zeile=1&gdz_unt_zeile=13&gdz_user_id=0, -> UTM32, Ebenen, ArcInfo-SHAPE). The polygons are NUTS DE124 and DE128 from file vg250_KRS.

Is there any way to dissolve these enclave-polygons with those other "normal" polygons in QGIS (I also tried the editor but it does not help)?

Answer 1

Premise: there is probably a typo in your original question since the features seem to be those having the "NUTS" field equals to 'DE121' and 'DE124'.

---

I firstly selected the two features of interest through a simple filtering expression:

"NUTS" = 'DE121' OR "NUTS" = 'DE124'

Then, I ran the Save selected features algorithm from the Processing Toolbox and I obtained a polygon containing two features:

Finally, I ran the Dissolve algorithm. If I leave the Dissolve all (do not use fields) checked, I can confirm your issue:

Instead, if I leave the Dissolve all (do not use fields) unchecked and choose any field storing a common value for both features (e.g. the "ADE" field), I get the expected result: