Using Prepared Statements with ORDER BY Parameters

When building SQL queries, especially those that involve dynamic parameters, it’s essential to use prepared statements to prevent SQL injection attacks. One common challenge when using prepared statements is dealing with ORDER BY parameters.

In this article, we’ll explore how to set ORDER BY params using prepared PDO statements and provide a working example.

Understanding Prepared Statements

Before diving into the solution, let’s quickly review how prepared statements work. A prepared statement is a precompiled SQL query that can be executed multiple times with different parameter values. The PHP PDO extension uses this feature to help prevent SQL injection attacks by separating the SQL code from user-input data.

When using prepared statements, you create a statement object and bind parameters to it using the bindParam or execute methods. The actual SQL query is not executed until you call the execute method.

The Problem with ORDER BY Parameters

The question we’re addressing arises when trying to use prepared statements for ORDER BY clauses, which typically involve dynamic parameter values. The problem is that the PDO extension cannot automatically escape these parameters because they are part of the SQL keyword sequence.

For example, if you try to use a variable as an ORDER BY parameter like this:

$stmt = $db->prepare("SELECT * from table WHERE column = :my_param ORDER BY :order");

The $order variable will not be properly escaped, and you’ll end up with a SQL query that looks something like this:

SELECT * from table WHERE column = ? ORDER BY ?

This is not a valid SQL query, as the ORDER BY clause requires a valid identifier (like a column name).

The Solution: Hardcoding Operators/Identifiers

Unfortunately, there isn’t a built-in constant for escaping these parameters. One common solution is to hardcode the operators and identifiers manually, like in the following example:

$orders = array("name", "price", "qty");
$key = array_search($_GET['sort'], $orders);
$order = $orders[$key];
$query = "SELECT * from table WHERE is_live = :is_live ORDER BY $order";

This approach requires you to know the valid operators and identifiers for your ORDER BY clause in advance. While it may seem inconvenient, it’s a common solution when working with prepared statements.

Whitelisting Helper Function

To reduce the amount of code that needs to be written and make the solution more robust, we can create a whitelisting helper function:

function white_list($value, $allowed_values, $error_message) {
    if (!in_array($value, $allowed_values)) {
        throw new Exception($error_message);
    }
    return $value;
}

$order = white_list($order, ["name", "price", "qty"], "Invalid field name");
$direction = white_list($direction, ["ASC", "DESC"], "Invalid ORDER BY direction");

$sql = "SELECT * from table WHERE column = ? ORDER BY $order $direction";
$stmt = $db->prepare($sql);
$stmt->execute([$is_live]);

This function checks if the provided value is in the list of allowed values and throws an exception if it’s not. This approach ensures that only valid values are used for your ORDER BY clause, helping to prevent errors and potential security vulnerabilities.

Conclusion

Using prepared statements with ORDER BY parameters requires some extra care when dealing with dynamic parameter values. While there isn’t a built-in constant for escaping these parameters, you can use hardcoded operators/identifiers or create a whitelisting helper function to make the process more manageable. By following these best practices and taking the necessary precautions, you can help ensure that your SQL queries are secure and reliable.

Example Use Case

Suppose we’re building an e-commerce application with a product listing page. We want to allow users to sort products by different columns (e.g., price, name, quantity) and in either ascending or descending order. Here’s how we could use the whitelisting helper function to handle these ORDER BY parameters:

if ($_GET['sort'] == "price") {
    $order = "price";
} elseif ($_GET['sort'] == "name") {
    $order = "name";
} else {
    throw new Exception("Invalid sort column");
}

if ($_GET['direction'] == "ASC") {
    $direction = "ASC";
} elseif ($_GET['direction'] == "DESC") {
    $direction = "DESC";
} else {
    throw new Exception("Invalid direction");
}

$query = "SELECT * from products WHERE is_active = :is_active ORDER BY $order $direction";
$stmt = $db->prepare($query);
$stmt->execute([$is_active]);

In this example, we’re using the whitelisting helper function to ensure that only valid sort column and direction values are used for our SQL query.

Last modified on 2023-08-09