php - 函数 SQL SUM 查询

Question

我有一张表格，上面有这样的数字:

 ______________________________________
| axle_id | train_id | axle | distance |  
|_________|__________|______|__________|   
|   1     |     1    |   1  |    20    |
|   2     |     1    |   2  |    50    |
|   3     |     1    |   3  |   200    |
|   4     |     1    |   4  |    50    |
|   5     |     1    |   5  |   200    |
|   6     |     1    |   6  |    50    |
|   7     |     1    |   7  |   200    |
|   8     |     1    |   8  |    50    |
|_________|__________|______|__________|

现在我想对它们进行计数并使用 SUM 进行计算。我现在拥有的代码:

function count_axles($id) {
        $sql = "SELECT sum(distance)/(25000)*(100) as totaldistance from axle WHERE train_id = :train_id";
        $sth = $this->pdo->prepare($sql);
        $sth->bindParam(":train_id", $id, PDO::PARAM_STR);
        $sth->execute();
        return $sth->fetchAll();
    }

我通过以下方式调用此函数:

<?php
        $count_axle = $database->count_axles($_GET['train_id']);
?>
<?php      
    foreach($count_axle as $counting){ 
    }

        echo $counting['totaldistance'];
?>

现在输出是正确的。
(3.2800000000000002) 这是 25.000 的百分比
但我想添加 5000 个:

$sql = "SELECT sum(distance)+(5000)/(25000)*(100) as totaldistance from axle WHERE train_id = :train_id";

但是当我这样做时，输出会随机生成如下内容:840

为什么会这样，我该如何解决？

Answer 1

基础数学。你在做什么

                       5000
  sum(distance) + --------------
                     25000 * 100

当你真正想要的时候

  sum(distance)  + 5000
  ------------------------
        25000 * 100

尝试

SELECT (sum(distance)+(5000))/(25000)*(100)
       ^--------------------^

相反。注意额外的括号。

记住旧的 BEDMAS 助记符:括号、指数、除法、乘法、加法、减法...