As you can see below I have 11 polygons in one shapefile, so in the attribute table there are 11 features.
As they all have the same attribute, is it possible to merge them into one feature in QGIS?
Another solution is the "Collect geometries" geoalgorithm, can be found under
Vector > Geometry Tools > Collect geometries.
Takes a vector layer and collects its geometries into new multipart geometries.
One or more attributes can be specified to collect only geometries belonging to the same class (having the same value for the specified attributes), alternatively all geometries can be collected.
All output geometries will be converted to multi geometries, even those with just a single part. This algorithm does not dissolve overlapping geometries - they will be collected together without modifying the shape of each geometry part.
The "Collect geometries" was applied with
"test" field as 'Unique ID fields'