php - 使用MySQL/PHP对列求和

我的问题是，当我在浏览器中运行代码时，为什么HW1 / HW2 / HW3列有空白值？
Studentid和Sum列正确显示了代码。任何理想的解决方法？

<?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);
?>

最佳答案

由于未选择列，请尝试：

$result = mysqli_query($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>";
}

关于php - 使用MySQL/PHP对列求和，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/15722542/