Oracle has something like 44% market share and SQL Server has something like 20%. More and more companies are starting to believe that SQL Server can now scale so you will be seeing more and more shops with larger SQL Server based apps. DB2 is loosing market share (we just got rid of all of our DB2 stuff and moved it to SQL Server or Oracle here) and IBM doesn't seem to be focusing much on the mid to small RDBMS market.

Oracle keeps buying up tons and tons of software applications and most of them have a database behind the scenes which will probably ONLY run on an Oracle db so Oracle will hold onto and/or increase their RDBMS market share.