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.

Databases are very sexy? They're super interesting from programming/CS perspective for multiple reasons.

Have you heard of PostgreSQL, MariaDB, or SQLite? They have very high market share.