QGIS » QGIS Application

adding this options in ftools (or gdal tools) does not make a lot of sense anymore, better add them directly in processing.

To add this tool it can be used a sql only approach, via ogr2ogr/processing, so I should be able to give it a shot.

A possible query is

SELECT
ST_Union(geom) AS geom,
a.diss_field,
null AS oth_field,
SUM AS area_ha
FROM
input a
WHERE
EXISTS (SELECT id_0 FROM input b WHERE b.id_0 != a.id_0 AND (ST_Intersects(a.geom, b.geom))) GROUP BY a.diss_field
UNION
SELECT
geom,
diss_field,
oth_field,
area_ha
FROM
input a
WHERE
NOT EXISTS (SELECT id_0 FROM input b WHERE b.id_0 != a.id_0 AND (ST_Intersects(a.geom, b.geom)))

where "oth_field" is the field to be kept untouched in non dissolved features, "diss_field" is the attribute used to dissolve adjacent features and "area_ha" a numeric field where to eventually compute an aggregation stat in dissolve features (and to be kept untouched in non dissolved features).

A slightly different case is when not only physically isolated features are not meant to be dissolved, but also when features that have only neighbours with a different dissolve field are not meant to be dissolved.

SELECT
ST_Union(geom) AS geom,
a.diss_field,
null AS oth_field,
SUM AS area_ha
FROM
input a
WHERE
EXISTS (SELECT id_0 FROM input b WHERE b.id_0 != a.id_0 AND (ST_Intersects(a.geom, b.geom)) and b.diss_field = a.diss_field) GROUP BY a.diss_field
UNION
SELECT
geom,
diss_field,
oth_field,
area_ha
FROM
input a
WHERE
NOT EXISTS (SELECT id_0 FROM input b WHERE b.id_0 != a.id_0 AND (ST_Intersects(a.geom, b.geom)) and b.diss_field = a.diss_field)

3 pictures are attached to understand better.