在SQL命令中计算百分比

Question

I'm trying to refactor my code. I was returning all rows, and filtering the result set with php, but this is quickly becoming a performance issue.

Current code:

$percentOff = 80;

$stmt = $this->dbh->prepare("SELECT * FROM products WHERE Available=1 AND Merchant='Amazon'"); //Selects all from DB. VERY slow

            $finalResults = array();
            //Limit the results returned
            foreach($rows as $row){
                $totalSavings = ($row["LowestNewPrice"] - $row["LowestUsedPrice"]) / $row["LowestNewPrice"] * 100;
                if($totalSavings >= $percentOff){
                    $finalResults[] = $row;
                }
            }

Refactored SQL:

$stmt = $this->dbh->prepare("SELECT * FROM products WHERE Available=1 AND Merchant='Amazon' AND (LowestNewPrice - LowestUsedPrice / LowestNewPrice * 100) >= ?");
$stmt->bindValue(1, $percentOff, PDO::PARAM_STR);

This returns ALL from products . I only want to return results that are >= 80. Where did I go wrong?

DB

| LowestNewPrice           | int(11)      | NO   |     | NULL                |       |
| LowestUsedPrice          | int(11)      | NO   |     | NULL                |       |

Answer 1

1. You are missing brackets around LowestNewPrice - LowestUsedPrice
2. As Luke mentioned, you may also be running into integer division. Multiply with 100 before you divide.