SQL is a functional programming language. No other imperative programming model will be cleaner or clearer in expressing intent. But, you have to understand SQL to begin with.
Did you mean to say declarative? That’s what makes it clear in intent isn’t it? That it’s declarative. Whether or not SQL is also functional is orthogonal to that isn’t it?
I'd consider INSERTs and UPDATEs to technically be "assignment", even if they're very different from how other languages do it.
Some SQL dialects do support both traditional variable assignment and iteration for those cases where an iterative/imperative approach makes more sense than trying to shoehorn the problem into something set-based / declarative. Some limit them to stored procedures (e.g. Postgres, and AFAICT Db2), while others allow them pretty much anywhere (e.g. SQL Server / T-SQL).
While there's no formal definition of functional programming (that I know of at least), SQL doesn't support higher order functions, which are fundamental to all functional programming languages. That said, it's definitely declarative.
