Nested JSON data using MySQL and PHP for AngularJs | ng-options

Could anyone please tell me how to create the following nested JSON data using MySQL and PHP in codeigniter.

I want my data in the given format.

$data =  {
    'India': {
        'Andhra Pradesh': ['Vijayawada', 'Guntur', 'Nellore', 'Kadapa'],
        'Madhya Pradesh': ['Hyderabad', 'Warangal', 'Karimnagar'],
    },
    'USA': {
        'San Francisco': ['SOMA', 'Richmond', 'Sunset'],
        'Los Angeles': ['Burbank', 'Hollywood']
    },
    'Australia': {
        'New South Wales': ['Sydney', 'Orange', 'Broken Hill'],
        'Victoria': ['Benalla', 'Melbourne']
    }
};

My Model code is below:

public function getAll()
    {
        $query = $this->db->query("SELECT
        `country`.`countryName`
        , `states`.`name` AS `stateName`
        , `cities`.`cityName`
        FROM
        `tablename`.`states`
        INNER JOIN `tablename`.`country` 
            ON (`states`.`country_ID` = `country`.`ID`)
        INNER JOIN `tablename`.`cities` 
            ON (`cities`.`state_id` = `states`.`ID`);");

        return $query->result();

    }

And Controller code is given below :

public function getAllData()
{
    //get All details
    $this->load->model('Shiksha_model','locations');
    $data = $this->locations->getAll();
    echo json_encode($data);
}

So I am gating the ouput as given below :

[
  {
    "countryName": "INDIA",
    "stateName": "West-Bengal",
    "cityName": "Kolkata"
  },
  {
    "countryName": "INDIA",
    "stateName": "Bihar",
    "cityName": "Purnia"
  }
]