At a company I used to work for the application used a somewhat proprietary files system based "database" utilizing a DISAM setup. .dat and .idx files and either file type was prone to corruption as soon as it would hit a certain size. It was fast though. Market demand was what really caused us to make the move to supporting either an Oracle or SQL Server back end and this was in the 6.5 and 7x of Oracle days. Once we did that I was thrown in to the database world where I had to be knowledgeable about both Oracle and SQL Server because we now had to code for both. I found all of it fascinating and I worked closely with the DBA we had at the time. Eventually he left the company and I moved in to his role. I left that company and moved in to the role of a production DBA for both Oracle and SQL Server with a focus on Oracle as that company was in the process of a total redo of the application that was the heart of the business.

That company didn't make it so the next move still involved daily work with both platforms but more for SQL Server as clients chose that due to the lower cost. Eventually most Oracle activity tapered of and tended to be more dev work and since my Oracle boxes were dev platforms there wasn't a need to keep my Oracle skills sharp so the focus was really all about SQL Server since I actually had production boxes on that platform. For years I was the DBA not just for our company but for many of our clients as the folks who ran Oracle tended to have deeper pockets and could afford to have a DBA or even a team of them on staff where the SQL Server clients typically did not.

Going on 15 years or so with SQL Server and I'm glad I stuck with it. I still find it fun to work with and it helps a lot that a DBA is always in need so finding a job isn't a problem.