Create an umbrella concept – say taggables. Both maps and video clips are taggables. In OO terms they’d implement the taggable interface or be subclasses of the taggable class. Here comes the messy part:

Create a table called taggables which only contains one column – it is the primary key (obviously) and contains a list of taggable uids. You then need a mapping table from each kind of taggable object to your taggables table (this is analogous to the code you’d have to write for each implementation of the taggable interface). For example a table called taggable_maps which has two columns – both foreign keys – one from maps and one from taggables. The same thing would exist for videos. You can then create your tag tables against that UID in the taggables database.

Possibly you could do this with a column in the videos table and in the maps table storing the tagableness ID – but I’ve not got time to do the necessary thinking to ensure that works in all cases. The advantage of seperate tables is that you can add taggableness without altering tables.

The only thing your database wouldn’t do for you is prevent a map and a video being linked to the same taggable ID.