UNION query with codeigniter's active record pattern

CodeIgniter's ActiveRecord doesn't support UNION, so you would just write your query and use the ActiveRecord's query method.

$this->db->query('SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2');

bwisn's answer is better than all and will work but not good in performance because it will execute sub queries first. get_compiled_select does not run query; it just compiles it for later run so is faster try this one

$this->db->select('title, content, date');
$this->db->where('condition',value);
$query1= get_compiled_select("table1",FALSE);
$this->db->reset_query();

$this->db->select('title, content, date');
$this->db->where('condition',value);
$query2= get_compiled_select("table2",FALSE);
$this->db->reset_query();

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