My collections consists of four image databases on four different file systems with subfolders in 3 levels. There is no view in digikam so far, to select all images e.g. of one of these databases (subfolders).

digiKam only adresses images on the top level of the selected folder, and does not support any operations if a (sub)folders has no images on top level.

Note: It is a great strength of digikam to support multiple file oriented asset locations - consequently a selection view should be provided to filter all contained assets (recursive, or to the nth level).