If you're SaaS, enterprise doesn't necessarily care what database you're using -- and some of them may be a polyglot even internally. (My dayjob is a SaaS that's old enough to have started on Oracle in part because we thought it would make us look "serious" to enterprise clients, which include some fairly large financial institutions. We just finally turned off the last Oracle server, after switching to Postgres; no one cared.)
Unfortunately, you can't count on the enterprise's pre-existing DB to support all of ANSI SQL well. Oracle in particular deviates from ANSI in a whole bunch of well-known ways (messed-up date types, treating the empty string, '', as the same thing as NULL, etc., etc.), which are almost certain to trip up SQL which hasn't been tested against it.
