That hasn't matched my experience. I read a fundamentals book on it, fully; a practical one (think it was T-SQL fundamentals, which was 90% ANSI sql). I did the problems, some were hard. Then its just kind of stuck around in my head, now nearly 15 years later. I use it often and am continuously shocked to understand it better than some of my colleagues, still, since I rarely use it. It also seems to have infected how I think, such that I'm often thinking in terms of SQL (or I guess, set theory really) when I"m reasoning about data and processing it. That's likely why it sticks, its just not that far removed from the operations that are happening (at the basic level), and then also not that complex. You aren't making new abstractions or layers with it, its a pretty limited set of features ultimately, and generally speaking it changes little if at all over time. Its great in that way, especially in this everything-changing-constantly industry.