Challenge 2: Type a Query Builder Result — Possible Solution
====================================================================

interface Database {
  products: {
    id: number;
    name: string;
    price: number;
    category: string;
  };
}

// A minimal typed "select columns" helper — the same idea a real query
// builder like Kysely applies at much larger scale.
function selectColumns<
  TTable extends keyof Database,
  TColumns extends keyof Database[TTable]
>(
  _table: TTable,
  columns: TColumns[]
): Pick<Database[TTable], TColumns> {
  // (Real implementation would run SQL; this signature is the point of the
  // exercise — the return type is DERIVED, not hand-written.)
  return {} as Pick<Database[TTable], TColumns>;
}

// --- Usage ---

const result = selectColumns("products", ["name", "price"]);
// result is inferred as { name: string; price: number } —
// NOT the full products row, and NOT `any`.

console.log(result.name);   // ✅ compiles — string
console.log(result.price);  // ✅ compiles — number
// console.log(result.category); // ❌ compile error: "category" was never selected

// A typo in the table or column name is also caught immediately:
// selectColumns("prodcuts", ["name"]);       // ❌ "prodcuts" is not keyof Database
// selectColumns("products", ["nmae"]);       // ❌ "nmae" is not keyof Database["products"]

WHY THIS WORKS
--------------
- `TTable extends keyof Database` restricts the first argument to an actual
  table name defined on Database — a typo fails at compile time instead of
  silently querying nothing.
- `TColumns extends keyof Database[TTable]` does the same for column names,
  scoped to whichever table was selected.
- `Pick<Database[TTable], TColumns>` builds the return type by picking only
  the requested columns off the full row type — exactly mirroring how a
  real query builder narrows the result shape to what was actually selected,
  the same idea Kysely and Prisma's `select` option use under the hood.