After a database migration from ... 5? to 8 all tags have vanished.
The migration tools sets the parent ids of those tags to "-1" instead of "0".

Reproducible: Always

Steps to Reproduce:
1. migrate database from old_dbname to new_dbname (mysql external to mysql external)
2. enter configuration
3. set new_dbname as current database

Actual Results:  
tags are neither shown in tag manager nor the images are tagged.

Expected Results:  
tags are available in tag manager and shown on images.

I've been using digikam for ages now without ever upgrading my database. I had to do the following steps to even convert the database successfully:

create table deleteme (id int(11) not null);
insert into deleteme (id) select id from Images where album not in (select id from Albums);
delete from Images where id in (select id from deleteme);
drop table deleteme;
delete from ImageTags where imageid not in (select id from Images);
delete from ImageHaarMatrix where imageid not in (select id from Images);
delete from ImageInformation where imageid not in (select id from Images);
delete from ImageMetadata where imageid not in (select id from Images);
delete from VideoMetadata where imageid not in (select id from Images);
delete from ImagePositions where imageid not in (select id from Images);
delete from ImageComments where imageid not in (select id from Images);
delete from ImageCopyright where imageid not in (select id from Images);
delete from ImageProperties where imageid not in (select id from Images);
delete from ImageHistory where imageid not in (select id from Images);
delete from ImageTagProperties where imageid not in (select id from Images);
delete from ImageRelations where subject not in (select id from Images) or object not in (select id from Images);
delete from ImageTags where tagid not in (select id from Tags);

and after running the migration tool, I got my tags back again using:
update Tags set pid = 0 where pid = -1 and name not like '_Digikam_%';