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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.

Explanation

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').

Example:

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.

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    I can probably help, but are you able to condense your question down into just a clear and short question so it is easier to understand what you are asking? Perhaps just start with the basic question that you mention right at the start. Could the second question, that you mention is optional, be asked separately after you have received an answer to the basic question? Stack Exchange works best if your questions can be concise, not chatty, and useful to others searching the site, so think how best you can achieve this. – Martin Hügi Mar 22 '17 at 18:36
  • Oke I can do that I will edit this now. Just give me a moment. – ThunderSpark Mar 23 '17 at 6:56
  • Great, I'll take a look in a bit then. – Martin Hügi Mar 23 '17 at 7:33
  • It is done I tried to keep the question short, and removed the optional part. But I still kept some things because I needed to explain what I exactly I wanted and to show that I searched for a similar question. – ThunderSpark Mar 23 '17 at 7:39
  • I have understood better now. It's not something that I know exactly what to do myself yet, but this is a possible duplicate to another question gis.stackexchange.com/questions/153982/… – Martin Hügi Mar 23 '17 at 7:46

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