Filter By Dates in SQL

I have a somewhat confusing problem.

First: I want to know how many days of payment correspond to a person according to their assistance within the current period.

Table usuarios :

id  Nombre fecha_ingreso
1   Juan   01-01-2018
2   Pedro  01-01-2018

Table asistencias , where 1 is assistance and 0 is missing:

id asist fecha
1  0     02-08-2018
1  1     03-08-2018
2  1     02-08-2018
2  1     03-08-2018

My table periodo contains the following info:

idp  diai        diaf
1    30-07-2018  12-08-2018

My script (working correctly, Note: this is in SQL server):

$conn ~~ aqui se conecta a la base de datos ~~
$sql = "SELECT * FROM usuarios ORDER BY id ASC";
$result = sqlsrv_query($conn, $sql);
while ($usuario = sqlsrv_fetch_array($result)) {
$id=$usuario['id'];

  $sql_periodo_inicial = sqlsrv_query($conn, "SELECT top 1 diai FROM periodo order by diai DESC");
   if($t_periodo_c=sqlsrv_fetch_array($sql_periodo_inicial)) {
                                $t_perido_diai=$t_periodo_c['diai'];

                            } 
  $sql_periodo_inicial = sqlsrv_query($conn, "SELECT top 1 diaf FROM periodo order by diaf DESC");
   if($t_periodo_c=sqlsrv_fetch_array($sql_periodo_inicial)) {
                                $t_perido_diaf=$t_periodo_c['diaf'];
                            }  


$sql_asistencia_asistidas2 = "SELECT * FROM asistencia 
                where id=$id 
                and fecha >= '$t_perido_diai' 
                and fecha <= '$t_perido_diaf' 
                and asistencia = '1'";
$params = array();
$options =  array( "Scrollable" => SQLSRV_CURSOR_KEYSET );
$stmt2 = sqlsrv_query( $conn, $sql_asistencia_asistidas2 , $params, $options );
$dias_trabajados_ttAsist = sqlsrv_num_rows( $stmt2 ); 

echo $dias_trabajados_ttAsist;
}

With this in my table it correctly shows me the results Juan 1 attendance and Pedro 2 within the current period

Question

Second : If Andrés ( id=3 ) entered after the cut-off date (' diaf' = 12 ) but before the fortnight that would be day 15 for example on August 13, 2018.

This person would have 2 days worked that will not be paid on day 15 but will be added to his next fifteen of day 30.

What should I add to my query to find Andrew's additional days?:

$sql_asistencia_asistidas2 = "SELECT * FROM asistencia 
                where id=$id 
                and fecha >= '$t_perido_diai' 
                and fecha <= '$t_perido_diaf' 
                and asistencia = '1'";

This is so that in my table you can automatically generate this value, but more important to be able to identify if there is an error when registering users.