That's ... probably not the best example, given the fact that there are a shedload of open-source databases of various types that have forced major commercial vendors like MSFT and ORCL into a corner. ORCL's moat is that they have a large portfolio of random solutions, are incumbent at a lot of organizations where switching costs are very high, and they have an exceptionally aggressive sales organization that doesn't seem to worry too much about legalities.