This is related to Limiting available values in dropdown field of QGIS form according to value selected in previous field?. Even though this has been marked as resolved the answer is not what I am looking for as this point towards creating a relation in the project setting.
But, I want to create a relation in 1, or more, database tables to a shapefile itself.
How can I link two, or three, value maps so that the drop down selection of the next field is limited by the value in the previous field? The choices are in a list bound to a database.
The reason why I need it in a choice table list instead of the shapefile itself is because the choice table needs to be updated.
For example I have a list to import from excel that has some value that are repeated, but they are distinct because of their subtypes, as illustrated below.
Sometimes there is a subtype, and sometimes not (assumed is 'NULL').
Begeleidingssnoei - achterstallige snoei
Begeleidingssnoei - bemantelingssnoei
Begeleidingssnoei - beschadigde takken
Onderhoudssnoei - innemen
Onderhoudssnoei - probleemtakken
Onderhoudssnoei - regressie
Because the list is for inventory fieldwork it will not be practical to have these values combined in a single list.
So I want to split it so the value map setting sets the drop down choice list in the next field.
But, importantly these value/s in this next field are different for each previous value.