sql查询中仅保留一列

Question

我想使用CI内置函数构建SELECT查询，但是查询总是 每行仅返回一列。

$this->db->select('L.DeviceId','U.FirstName','U.LastName','L.Latitude As lat','Location.Longitude As lng','L.ServerDateTime');
$this->db->from('Location L');
$this->db->join('User_Device UD','L.DeviceId = UD.DeviceId');
$this->db->join('User U','UD.UserId = U.UserId');
$this->db->where('U.ClientId',1);
$data = $this->db->get();

我期望类似

[{'DeviceId':'1','FirstName':'XYZ','LastName':'LMN',21.03,71.23,'2019-05-07 20:00:00'},...]

但是得到

[{'DeviceId':'1'},..]