你不需要使用join。你可以通过Union来实现。

$condition_array = 'emep1.is_deleted ="0")';
$this->db->select("emep1.emp_id, emep1.emp_name, emep1.salary");
$this->db->from('tbl_emp as emp1');
$this->db->where($condition_array);
$query1 = $this->db->get_compiled_select();

$condition_array = 'emep2.is_deleted ="0")';
$this->db->select("emep2.emp_id, emep2.emp_name, emep2.salary");
$this->db->from('tbl_emp2 as emp2');
$this->db->where($condition_array);

//Group by if you needed
$this->db->group_by('emp1.emp_id');
$query2 = $this->db->get_compiled_select();

$query = $this->db->query("select * from (" . $query1 . " UNION " . $query2 . ") as combine_table_data ORDER BY emp_id desc");
return $query->result_array();

字符串
注意：在select语句中，您应该有相同的字段。