Write union query in codeigniter style

问题:

How can I write the following query in Codeigniter style.

SELECT COUNT(`id`) AS reccount
  FROM 
    (SELECT `id` FROM table1 
     WHERE tid= '101' AND `status` =  1
     UNION ALL
    SELECT `id` FROM table2 
     WHERE tid= '101' AND `status` =  1
     UNION ALL
    SELECT `id` FROM table3
     WHERE tid= '101' AND `status` =  1) t

I have used the following way to execute it.

Is it the only correct way or do you have any suggestion to improve it?

 $q = $this->db->query(SELECT COUNT(`id`) AS reccount
                        FROM 
                        (SELECT `id` FROM table1 
                         WHERE tid= '101' AND `status` =  1
                         UNION ALL
                         SELECT `id` FROM table2 
                         WHERE tid= '101' AND `status` =  1
                         UNION ALL
                         SELECT `id` FROM table3
                         WHERE tid= '101' AND `status` =  1) t ");

回答1:

Since CodeIgniter 3 it's been introduced in Active Record the function get_compiled_select() that gives the query string without actually executing the query.

This allows @MDeSilva method to use less resources, being adapted as follows:

function get_merged_result($ids){                   
    $this->db->select("column");
    $this->db->distinct();
    $this->db->from("table_name");
    $this->db->where_in("id",$model_ids);
    $query1 = $this->db->get_compiled_select(); // It resets the query just like a get()

    $this->db->select("column2 as column");
    $this->db->distinct();
    $this->db->from("table_name");
    $this->db->where_in("id",$model_ids);
    $query2 = $this->db->get_compiled_select(); 

    $query = $this->db->query($query1." UNION ".$query2);

    return $query->result();
}

回答2:

You can use CI to generate a union query. However, latest versions made this much harder than before.

DB has a method called _compile_select, in previous versions of CI it was public, however now it is protected so you can't just call $this->db->_compile_select() from your controller. In order to do this properly one could:

1. Create custom loader class to be able to extend core/database classes (i.e. load MY_DB_active_record instead of CI_DB_active_record).

2. Create custom activerecord class, with just one method:

   public function compile_select() {
       return $this->_compile_select();
   }
   

3. In your controller, create all necessary queries, compile them into a string array using our public method compile_select()

4. Join the array into single query: '(' . implode(') UNION (', $queries) . ')'. You can also wrap this into a separate method inside your custom AR class.

回答3:

function get_merged_result($ids){                   
    $this->db->select("column");
    $this->db->distinct();
    $this->db->from("table_name");
    $this->db->where_in("id",$model_ids);
    $this->db->get(); 
    $query1 = $this->db->last_query();

    $this->db->select("column2 as column");
    $this->db->distinct();
    $this->db->from("table_name");
    $this->db->where_in("id",$model_ids);

    $this->db->get(); 
    $query2 =  $this->db->last_query();
    $query = $this->db->query($query1." UNION ".$query2);

    return $query->result();
}