SAO got adapted into anime in such a weird way. The first season is not even just the first novel. Instead they added in a bunch of side stories, that were written later, as filler. Season 2 is comprised of the next 2 novels, and then the 4th novel didn't even get a show of it's own, just a movie. And then came the Alice story arc... that thing is ridiculous... I'm pretty sure it has well over a dozen novels. Don't even know if he finished writing it. So far they've adapted it into 2 seasons. But I've stopped watching before them. All in all, it just seems that the adaptations just got more and more compressed as it went on.