不同/连接两个表

Question

商店表：

+--+-------+--------+ 
    |id|name |date | 
    +--+-------+--------+ 
    |1 |x  |March 10| 
    +--+-------+--------+ 
    |2 |y  |March 10| 
    +--+-------+--------+ 

类别表：

+--+-------+ 
|id|title | 
+--+-------+ 
|1 |tools | 
+--+-------+ 
|2 |foods | 
+--+-------+ 

店类别表（shop_cats）：

+--+-------+--------+ 
|id|shop_id|cat_id | 
+--+-------+--------+ 
|1 |1  |1  | 
+--+-------+--------+ 
|2 |1  |2  | 
+--+-------+--------+ 

我想按类别商店（类别存储在$ cat数组中）

 $this->db->select('shops.*'); 
    $this->db->from('shops'); 
    if(!empty($cat)) 
    { 
     $this->db->join('shop_cats' , 'shop_cats.shop_id = shops.id'); 
     $this->db->where_in('shop_cats.cat_id' , $cat); 
    } 


    $this->db->limit($limit , $offset); 
    $res = $this->db->get(); 

我的问题是，它返回重复的结果 例如，在该表中

+--+-------+--------+ 
|id|shop_id|cat_id | 
+--+-------+--------+ 
|1 |1  |1  | 
+--+-------+--------+ 
|2 |1  |2  | 
+--+-------+--------+ 

，如果我想用（1,2）I类获得ID = 1号店的商店，两次。 我希望它只返回每个商店一次而没有任何重复。

我已经尝试过通过

if(!empty($cat)) 
     { 
      $this->db->join('shop_cats' , 'shop_cats.shop_id = shops.id'); 
      $this->db->group_by('shop_cats.shop_id'); 
      $this->db->where_in('shop_cats.cat_id' , $cat); 
     } 

使用组也没有工作，我也试着

if(!empty($cat)) 
     {   $this->db->select('DISTINCT shop_cats.shop_id'); 
      $this->db->join('shop_cats' , 'shop_cats.shop_id = shops.id'); 
      $this->db->where_in('shop_cats.cat_id' , $cat); 
     } 

，但我得到语法错误！

Answer 1

尝试

$this->db->distinct('shops.*'); 
$this->db->from('shops'); 
$this->db->join('shop_cats', 'shop_cats.shop_id = shops.id', 'left'); 
$this->db->where('shop_cats.cat_id', $cat); 
$this->db->limit($limit , $offset); 
$res = $this->db->get();