Please read the FAQ relate to Windows:
And also the User Manual is extremely useful if you search for answers:
Ansewring your original question: database file is located in the configuration directory by default.
Also the command line works just on other platforms, just every output is redirected to the log file , even if you start darktable with
--help parameter - That’s not very fortunate, and we might change that soon to be more user friendly.
--library command line switch is also working on Windows. I have created a separate “catalog” and I can start darktable by
"C:\Program Files\darktable\bin\darktable.exe" --library C:\Users\[username]\AppData\Local\darktable\catalog1.db just fine either from the command line or creating a new shortcut at the Start Menu. If the database does not exist, darktable will create a new empty database (library) for you.