php - 使用 MySQL/PHP 对列求和

Question

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。任何理想的如何解决这个问题？

<?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

只需将您的查询更改为此

"SELECT studentid,hw1, hw2, hw3, SUM(hw1+hw2+hw3) as hw
FROM grade
GROUP BY studentid"

Answer 2

我不确定，因为你的问题写得不好。但我认为你只需要这样做来获取值：

选择学生ID,hw1,hw2,hw3,SUM(hw1+hw2+hw3)

Answer 3

因为您没有选择列，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>";
}