Functional SQL: https://www.codewars.com/kata/545434090294935e7d0010ab
This example recreates a functional SQL query engine in JavaScript, translating a Codewars kata into a working API. The code defines a `query` object with chainable methods like `select`, `from`, `where`, `groupBy`, `having`, and `orderBy`, mirroring SQL clause semantics. It performs cross joins across multiple arrays, applies filtering via nested callback arrays, and builds nested group structures using plain objects and recursive traversal. The final `execute` method returns mapped rows, demonstrating how relational query concepts can be implemented without external libraries, relying solely on native array methods and object literals.