当我使用else语句时，我无法回显Invalid

I cant view Invalid, any suggestion to make it run?

    <table border="1">
<tr>
    <th>First Name</th>
    <th>Last Name</th>
    <th>Strand</th>
    <th>Section</th>
</tr>   

<?php
 $conn=mysqli_connect("localhost","root","","search");
 $set=$_POST['search'];
 if ($set > 0) {
 $show="SELECT * FROM search_student WHERE fname='$set'";
 $result=mysqli_query($conn,$show);

 while ($rows=mysqli_fetch_array($result) ) { 
    echo  "<tr>";
    echo  "<td>";
    echo $rows['fname'];
    echo  "</td>";
    echo  "<td>";
    echo $rows['lname'];
    echo  "</td>";
    echo  "<td>";
    echo $rows['strand'];
    echo  "</td>";
    echo  "<td>";
    echo $rows['section'];
    echo  "</td>";
    echo  "<tr>";
    echo "</br>";
 }
 }

The else statement here doesn't work

  else{
 echo "invalid";
 }

 ?>
 </table>

    if(mysqli_num_rows($result)>0){
        while ($rows=mysqli_fetch_array($result) ) { 
        echo  "<tr>";
        //etc

       }
     }
     else
     {
      echo "invalid";
     }