Reading up on the history parts in the Wikipedia article, the German language version presents it as if SQL was the Oracle (Relational Software Inc) implementation of IBM's Sequel, the English page says the name was shortened due to trademark issues, before mentioning any implementation outside IBM. That would make Sequel and SQL the same thing, just rebranded early on. It also lists a clever (b)acronym and two other reasons for why they called it sequel before shortening, so it seems like there's a decent amount of mystery around the name. Almost as if it happened before the invention of writing.