This question has three folds:

1, Suppose I have a vector layer in qgis with an attribute field of “type” (string), how to limit the acceptable field value? say I only accept value as one of (“A”, “B”, “C”)?

2, How to configure this so that whenever I add a new feature in editing mode, I can select from a dropdown list of (“A”, “B”, “C”) to fill column “type” for the new feature? Or it will be automatically set up after I set up part 1?

3, How to set this up when this layer is actually a postgis database layer? What do I need to set up from the database side?

thanks