codeigniter active records group_by returns random ordering of joined table record id

问题

I'm using Codeigniter, what I want to do is a query that returns joined table (item_gallery) gallery_id is lowest value. while main query ordered by desc of items post_date

the below code group_by select random value of item_gallery gallery_id. but i want the lowest value of item_gallery gallery_id.

  public function shopItems($id) {

    $this->db->select("*");
    $this->db->from('items');
    $this->db->join('item_gallery', 'items.id = item_gallery.item_id', 'left');
    $this->db->where('items.user_id', $id);     
    $this->db->order_by('items.post_date', 'Desc');
    $this->db->group_by("item_gallery.item_id");
    $query = $this->db->get();
    return $query->result();
}

database Structure

items table

|    id    |  slug  |  user_id  |  post_date |

|    12    |  test  |    111    |  12/5/2017 |

items Gallery

| gallery_id |  item_id  |     image     | 

|     121    |    12     |  profile.png  |   -- i want this record selected
|     122    |    12     |  gallery.png  | 

回答1:

Replace

$this->db->order_by('items.post_date', 'Desc');

With

 $this->db->order_by('items.id', 'Desc');

回答2:

public function shopItems($id, $limit, $start) {

date_default_timezone_set('Asia/Colombo');
$expire = date('Y-m-d');
$this->db->select("*,(SELECT image FROM item_gallery WHERE item_id = 
items.id ORDER BY gallery_id ASC LIMIT 1) AS profile_img");
$this->db->from('items');
$this->db->join('item_gallery', 'items.id = item_gallery.item_id', 'left');
$this->db->where('items.exp_date >', $expire);
$this->db->group_start();
$this->db->where("items.status = 'yes'");
$this->db->or_where("items.status = 'edit'");
$this->db->group_end();
$this->db->where('items.user_id', $id);
$this->db->limit($limit, $start);
$this->db->order_by('items.post_date', 'Desc');
$this->db->group_by("item_gallery.item_id");
$query = $this->db->get();
return $query->result();
}

来源：https://stackoverflow.com/questions/44668853/codeigniter-active-records-group-by-returns-random-ordering-of-joined-table-reco