[qsort]

It doesn't improve the power of SQL, it's just syntactic sugar. Because of how the SQL syntax works, you'd have to do something like:

   WITH A AS (
        SELECT x, sum(y) AS z
          FROM SomeTable
      GROUP BY x
   )

   SELECT * FROM A WHERE z > 10

With an HAVING clause you can instead just tuck it after the GROUP BY clause.

Also, although it's not an issue these days given how good query planners are (any decent engine will produce exactly the same query plan with a subquery or an having clause, it's indexes that fuck up stuff), but you're signaling that the filter happens "at the end".

It's like having both "while" and "for" in a programming language. Technically you don't need it, but it's for humans and not for compilers.