Let's not erase the work of Stonebraker et al whose efforts through the 70s were a lot closer to Codd's relational algebra than SQL. Ingres's query language, Quel, springs to mind: being directly based on the relational calculus it was far more readily composable than SQL (which might be charitably described as "borrowing ideas from the relational model") was or is.
As with the videotape format war, the eventual industry standard was decided commercially: Oracle was by far the most aggressive, and promoted SQL.
Yes, we can have relational databases without SQL, there are better ways for composability. Some might argue that SQL itself isn't a mess, but it sure does help people make a mess of themselves.
As with the videotape format war, the eventual industry standard was decided commercially: Oracle was by far the most aggressive, and promoted SQL.