Write union query in codeigniter style - codeigniter

Question

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 ");

Answer 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();
}

Answer 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:
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).
Create custom activerecord class, with just one method:
public function compile_select() {
return $this->_compile_select();
}
In your controller, create all necessary queries, compile them into a string array using our public method compile_select()
Join the array into single query: '(' . implode(') UNION (', $queries) . ')'. You can also wrap this into a separate method inside your custom AR class.

Answer 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();
}