I don't know if an other user friendly way exist (an other way than a "expert" sql query

But, right now, it would be interesting to have a friendly way to clear all blank categories into the xnview db (ex: IPTC keywords not attached to any files but still into db) ... of course, it is already possible to select (or multi select) within the "category filter tab" the blank items (and then, with a right mouse click, and delete) ... but maybe into this same contextual menu, it would be interesting to have a way to automatically select all blank items ? (to have the ability to clear/delete these items in one step).