The next release has a new command-line function: calibredb list_categories
Documentation:
Sample output: calibredb list_categories -i
Spoiler:
> calibredb list_categories -i
Code:
category tag_name count rating
authors CATEGORY ITEMS 17 0.0
formats CATEGORY ITEMS 6 0.0
news CATEGORY ITEMS 2 0.0
publisher CATEGORY ITEMS 10 0.0
rating CATEGORY ITEMS 3 0.0
search CATEGORY ITEMS 4 0.0
#series CATEGORY ITEMS 1 0.0
series CATEGORY ITEMS 3 0.0
tags CATEGORY ITEMS 30 0.0
#text CATEGORY ITEMS 2 0.0
#textmult CATEGORY ITEMS 1 0.0
#tool CATEGORY ITEMS 1 0.0
Sample output: calibredb list_categories (edited)