Enterprises do use SQL in addition to Oracle, DB2, etc. We're a Fortune 200 and we have more SQL DBs than Oracle or the others combined. SQL is a good product and is quite robust and flexible. We use it in some pretty impressive ways. SQL has powered the NASDAQ for quite some time.
http://www.microsoft.com/en-us/sqlse...e-studies.aspx
I've been hearing about Linux taking over the world since the 90s. HP is driving their proprietary high-availability features into Windows and Linux, so don't expect HP-UX to be out there for more than about 5 years or so. Oracle doesn't seem to know what they're going to do with Solaris. IBM is expensive, so I've encountered more limited use of AIX/DB2.