$db = \Config\Database::connect();

//After setting up db conf in your .env file Pass this in constructor of your controller like

public $db;

public function __construct(){

$this->db=\Config\Database::connect();

}

//And Use it anywhere you want

public function myFunction(){

       $sql = 'SELECT * from mytable';

        $query = $this->db->query($sql);

        $result = $query->getResult();

}