Which schools are known for graduation level database research? Which schools across the world are best known for research being carried out in database field in recent times?
MIT is the home of Michael Stonebraker, probably the leading RDBMS researcher working in academia today. A lot of the research is done in private labs tho', e.g. CJ Date and Ted Codd did much of their early work at IBM.

I am missing a huge amount here as I am only really interested in RDBMS research; there's probably a lot more going on in OODBMS research.