在CodeIgniter中连接两个或多个表

I'm very new to the concept of joining two tables in CodeIgniter query. Can someone explain the meaning of each line of these codes, please? I don't understand them at all.

    $this->db->select('d.*, u.first_name, u.last_name');          
    $this->db->where('status', -1);        
    $this->db->join('users AS u','u.id = d.user_id');
    $this->db->order_by('d.date','desc');
    return $this->db->get('dtr AS d');

Just refer to codeigniter user guide here And i suggest if you are not compatible on doing that way try to code like this:

   $sql = "SELECT * FROM $table_name";
   $this->db->query($sql);

It is just the same way around.

Joins is not a CodeIgniter concept but a relational database concept. The SQL code of this query is :

SELECT d.*, u.first_name, u.last_name'
FROM dtr AS d 
INNER JOIN users AS u ON u.id = d.user_id
WHERE status = -1
ORDER BY d.date DESC

the query calls two table and joins them using a common key (user: id and dtr: id).