queries inside of a class

I’m building a plugin and I’m quite new to wordpress I have a class what has couple of methods what should execute database queries. The class look as it follows

class MyClass{

        private $wpdb;

        function __construct(){
            global $wpdb;

        }

        function query(){
        $wpdb->query( $wpdb->prepare("INSERT INTO $wpdb->wp_competitors

                        ( id, field_key, field_value ) VALUES ( %d, %s, %s )",1,

                        $field_key, $field_value) );

        }

     }

I would like to be able to use inside of my class wordpress native database class, how to do this?

class MyClass{
    function __construct(){
        global $wpdb;
        $this->db = $wpdb;
    }

    function query(){
       return $this->db->query( 
            $this->db->prepare("INSERT INTO {$this->db->wp_competitors} (id, ield_key, field_value) VALUES ( %d, %s, %s )",
            1, $field_key, $field_value) 
       );
    }
}