@cookie A bit of a threadcromancy but I also ran into some issues there and made the account to suggest fixes
Double vs single quotes got me but also after importing, loads of my playlists were empty. This is because not all of my stuff is within the Library DB. So on the statement where you're adding to the playlist_items table, there are two considerations:
As OP made a case, the filename column in Clementine is NULL for Library type entries, and you need to insert '' instead.
ADDITIONALLY, you DO want to insert also where type = 'File'.
I'm bad with SQL so i just ran the thing twice instead of looking up how to inline OR, but yeah, this needs to get fixed in the wiki too