mysql - PHP query fetching records from table2 to table 1 -

i got 2 tables

student

s_id, lastname, firstname, middlename 

takensubject

s_id, subjectcode, time 

i have code below using session display take subject takensubject display id want display full name of student using id takensubject code below show error warning: mysqli_num_rows() expects parameter 1 mysqli_result, boolean. can me correct query?

<?php      include'database.php';      $sescode = $_session['sessioncode'];      $sestime = $_session['sessiontime'];      $conn = mysqli_connect($server, $dbusername, $dbpassword, $database);      if (!$conn) {          die("connection failed: " . mysqli_connect_error());      }      $sql = "select * takensubject tb2, student tb1 tb2.s_id=tb1.s_id , schoolyear ='$sy' , semester ='$sem' , subjectcode='$sescode' , time='$sestime'";      $no = 0;      $result = mysqli_query($conn, $sql);      if (mysqli_num_rows($result) > 0) {          while ($row = mysqli_fetch_assoc($result)) {              $no++;              echo"<td>$no ).</td> ";              echo"{$row['s_id']}";              echo"{$row['lastname']}<br>";          }      } else {          echo "no record results";      }      mysqli_close($conn);  ?>

it keep showing warning:

mysqli_num_rows() expects parameter 1 mysqli_result, boolean

i guess query returning false. try this.

select tb1.*, tb2.*    takensubject tb2    inner join student tb1    on tb1.s_id= tb2.s_id    schoolyear ='$sy'     , semester ='$sem'     , subjectcode='$sescode' , time='$sestime';