users(*)
  ~> {country, "users":
        ~> {first_name, last_name}} as users_by_country

 WITH
  cte_1 AS (
    SELECT
      users.id AS id,
      users.first_name AS first_name,
      users.last_name AS last_name,
      users.email AS email,
      users.age AS age,
      users.status AS status,
      users.country AS country,
      users.created_at AS created_at,
      users.last_login AS last_login,
      users.balance AS balance
    FROM users
  ),
  cte_2 AS (
    SELECT
      cte_1.country AS country,
      COALESCE(
        JSON(
          '['
          || GROUP_CONCAT(
            CASE
              WHEN cte_1.first_name IS NOT NULL
              OR cte_1.last_name IS NOT NULL THEN JSON_OBJECT(
                'first_name',
                cte_1.first_name,
                'last_name',
                cte_1.last_name
              )
            END,
            ','
          )
          || ']'
        ),
        JSON('[]')
      ) AS users
    FROM cte_1
    GROUP BY
      cte_1.country
  ),
  cte_3 AS (
    SELECT
      COALESCE(
        JSON(
          '['
          || GROUP_CONCAT(
            CASE
              WHEN cte_2.country IS NOT NULL
              OR cte_2.users IS NOT NULL THEN JSON_OBJECT(
                'country',
                cte_2.country,
                'users',
                json(cte_2.users)
              )
            END,
            ','
          )
          || ']'
        ),
        JSON('[]')
      ) AS users_by_country
    FROM cte_2
  )
SELECT cte_3.users_by_country AS users_by_country FROM cte_3;

[
{
  "country": "Argentina",
  "users": [
    {
      "first_name": "Rachel",
      "last_name": "Lopez"
    }
  ]
},
{
  "country": "Australia",
  "users": [
    {
      "first_name": "Charlie",
      "last_name": "Brown"
    },
    {
      "first_name": "Jack",
      "last_name": "Jackson"
    },
    {
      "first_name": "Ben",
      "last_name": "Taylor"
    }
  ]
},
{
  "country": "Brazil",
  "users": [
    {
      "first_name": "Tina",
      "last_name": "Perez"
    }
  ]
},
{
  "country": "China",
  "users": [
    {
      "first_name": "Wendy",
      "last_name": "Chen"
    },
    {
      "first_name": "Xavier",
      "last_name": "Wang"
    }
  ]
},
{
  "country": "France",
  "users": [
    {
      "first_name": "Eva",
      "last_name": "Miller"
    },
    {
      "first_name": "Leo",
      "last_name": "Harris"
    },
    {
      "first_name": "Gina",
      "last_name": "Walker"
    }
  ]
},
]