pointer to feature comparisons, please

Hello List,

Short version: I want pointers to feature comparisons of Postgres vs
Oracle. Can the list help?

Long version:

I'm working with a student on a project for school. I'm trying to
teach "right" methods of thinking and doing things, such as making
the database/data model the authoritative source rather than adding
code to the application layer.

I originally had him code his project for Postgres, but for reasons
beyond our control we've had to move to Oracle. In designing the
schema we have need of a constraint that checks values in other
tables. The way that I currently know how to do this in Postgres is
with PLpgSQL functions. Then I add something like

CONSTRAINT away_team_is_playing CHECK ( NOT teamIsPlaying
( awayteamid, timeid ) )

to the table schema. No big deal, except that it seems Oracle can't
use anything other than a simple column constraint. He can't use any
custom functions like he could in Postgres, and we've yet to find a
solution to do what he needs.

I didn't immediately find anything last night on the postgresql.org
website, or a wider Google search.

So, motivation aside, what I'm wanting is a couple of pointers to
feature comparisons of Postgres vs Oracle. What else is going to
bite him while he works on this project? Would be handy to have this
reference since neither of us are really DB wizards. (Besides!
Isn't it good to tout what Postgres does better than it's
competition? :-) )

Thanks,

Kevin