sql inner连接两个表以通过日期范围查询获取数据

Question

I am trying to query data from two tables namely "tbl_tmpdtr" and "tbl_employee" using date range to compare in the tbl_tmpdtr field namely in_out datetime(). i have two input type="date" in my view as i pass it on my controller { $this->input->post('from_date'); $this->input->post('to_date') } to be used in my model check out the code below.

public function view_emp_dtr($a, $b)
{
    $q = $this->db->query('SELECT tbl_tmpdtr.id, tbl_tmpdtr.userid, date(tbl_tmpdtr.in_out) as in_out, tbl_tmpdtr.status, tbl_employee.emp_id, tbl_employee.emp_lname, tbl_employee.emp_mname, tbl_employee.emp_fname, tbl_employee.emp_gender, tbl_employee.civil_status, tbl_employee.dob, tbl_employee.street, tbl_employee.barangay, tbl_employee.city, tbl_employee.province, tbl_employee.emp_contact, tbl_employee.spouse_lname, tbl_employee.spouse_mname, tbl_employee.spouse_fname, tbl_employee.spouse_gender, tbl_employee.relationship, tbl_employee.address, tbl_employee.spouse_num, tbl_employee.jobtitle_id, tbl_employee.store_id, tbl_employee.username, tbl_employee.password, tbl_employee.hiredate, tbl_employee.emp_status, tbl_employee.Image FROM tbl_tmpdtr INNER JOIN tbl_employee ON tbl_tmpdtr.userid = tbl_employee.emp_id WHERE in_out<='$a' AND in_out>='$b' GROUP BY tbl_tmpdtr.userid');

    if($q->num_rows() > 0)
    {
        foreach($q->result() as $row)
        {
        $data[]= $row;
        }
    return $data;
    }
}  

my problem: the variable $a and $b are considered as columns;