使用MySQL/PHP的總和列

Question

我的問題是，當我在瀏覽器中運行代碼時，爲什麼HW1/HW2/HW3列有空白值？
Studentid和Sum列正確顯示代碼。任何理想的如何解決這個問題？

<?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

因爲你沒有選擇的列，請嘗試：

$result = mysqli_query($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>"; 
} 

Answer 2

我不確定，因爲你的問題寫得不好。但我認爲你只需要做到這一點得到的值：

SELECT studentid，HW1，HW2，HW3，SUM（HW1 + HW2 + HW3）

Answer 3

只是改變你的查詢到該

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