函数SQL SUM查询

Question

I have a table with numbers like this:

 ______________________________________
| 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    |
|_________|__________|______|__________|

Now i want to count them and make a calculation with SUM. The code i have right now:

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();
    }

And i call this function via:

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

        echo $counting['totaldistance'];
?>

Now the output is correct.
(3.2800000000000002) This is the percentage of 25.000
But i want to add 5000 like:

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

But when i do that, the output generates something randomly like: 840

Why does it do this, and how do i solve this?

Answer 1

Basic math. You're doing

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

when you really want

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

Try

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

instead. Note the extra brackets.

And remember the old BEDMAS mnemonic: brackets, exponents, division, multiplication, addition, subtraction...

Answer 2

Try with:

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

Division has preference over sum operations.