Oracle is a powerhouse in database management systems, but it’s hardly a monopolist. IBM revels in contriving figures that show it to have market share comparable to Oracle’s, and Microsoft has a very solid position as well. Smaller players like Teradata, Sybase, and MySQL are also thriving. And of course there’s a whole wave of newer DBMS companies, from Netezza on, showing that the DBMS industry isn’t even the secure oligopoly it appeared to be earlier this decade.
However, it’s certainly legitimate to define a product category of “real” DBMS that includes everything from MySQL on up, but not Microsoft Access and other low-end data management products. In that universe, while MySQL is a trivial addition to Oracle’s revenue, it’s a huge increment to Oracle’s unit market share. A merged Oracle/MySQL will dwarf the competition in ways that Oracle or MySQL alone don’t.
I can probably come up with business practices that could make things very hard on Oracle/MySQL competitors. E.g., Oracle could evolve MySQL in a direction that makes it sensible to put a MySQL transparency front end onto the Oracle DBMS. Add in a few specialized MySQL engines as they mature, and the whole thing could be a formidable family.
On the other hand — MySQL is open source now. There’s nothing, in theory, to keep the community from forking it.
I may well be overlooking something, but I haven’t found a compelling antitrust trigger on my first pass over the subject.