All parts of the content are tied to the siteid and siteguid. It is theoretically possible to move content from one site to another in the db by updating the siteid and siteguid on mp_Pages, mp_Modules but understand that the site map is cached so changes in the db aren't reflected in the menu until you clear the cache by recycle the app pool or touch web.config
be careful that you also adjust parentid and parentguid in mp_Pages so that the parent is not form a different site. Possibly other things also would need changing but those are the main things. roles might be different between sites so pages may end up with incorrect roles. userid and or userguid may be attached to the data and may be invalid if you change the site, etc.
I would generally recommend avoid doing this change and move the content by copy paste or re-creating it in the other site. You can try doing it in the db but you may miss something and your mileage may vary.
Hope that helps,
Joe