There are a lot of changes to the DB and how things may work between XF1 and XF2 -- the upgrade script to the first 2.0 release is nearly 3000 lines, for example. We could really only recommend using the XF2 APIs to work with things.
Saying that, a lot of the DB structure is very similar, but you really should be interacting (particularly when making changes to content) via XF rather than on the DB directly.