If you're asking about Liferay's own database, its tables and
columns: Here's
what you need to know about it, and how to change it.
If you're asking about your own, custom, database structures, you're
free to do whatever you want: There's nothing in Liferay dependent on
it - just make sure you update your own code.
If the database has been created through ServiceBuilder (but still
custom, in your own code), you might want to look into the upgrading
routines. They go together with deploying new version of a service and
can update the database according to your needs.
