使用MySQL / PHP汇总列

Question

My question is that why there are blank values for HW1/HW2/HW3 columns when I ran the code in the browser?.
Studentid and Sum columns displayed the code correctly. Any ideal how to fix this?

<?php

$result = mysqli_query($con,"SELECT studentid,SUM(hw1+hw2+hw3) 
FROM grade
GROUP BY studentid");

echo "<table border='1'>
<tr>
<th>StudentID</th>
<th>HW1</th>
<th>HW2</th>
<th>HW3</th>
<th>SUM</th>

</tr>";

while($row = mysqli_fetch_array($result))
 {
 echo "<tr>";
 echo "<td>" . $row['studentid'] . "</td>";
 echo "<td>" . $row['hw1'] . "</td>";
 echo "<td>" . $row['hw2'] . "</td>";
 echo "<td>" . $row['hw3'] . "</td>";
 echo "<td>" . $row['SUM(hw1+hw2+hw3)'] . "</td>";

 ;}

 echo "</table>";

 mysqli_close($con);
?>

Answer 1

Because you did not select the columns, try:

$result = mysqli_query($con, 'SELECT `studentid`, `hw1`, `hw2`, `hw3`, SUM(`hw1`+`hw2`+`hw3`) as `sum` FROM `grade` GROUP BY `studentid`');

while($row = mysqli_fetch_array($result)) {
    echo "<tr>";
    echo "<td>" . $row['studentid'] . "</td>";
    echo "<td>" . $row['hw1'] . "</td>";
    echo "<td>" . $row['hw2'] . "</td>";
    echo "<td>" . $row['hw3'] . "</td>";
    echo "<td>" . $row['sum'] . "</td>";
}