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sql – Are Dynamic Prepared Statements Bad? (with php + mysqli)

Posted by: admin July 12, 2020 Leave a comment

Questions:

I like the flexibility of Dynamic SQL and I like the security + improved performance of Prepared Statements. So what I really want is Dynamic Prepared Statements, which is troublesome to make because bind_param and bind_result accept “fixed” number of arguments. So I made use of an eval() statement to get around this problem. But I get the feeling this is a bad idea. Here’s example code of what I mean

// array of WHERE conditions
$param = array('customer_id'=>1, 'qty'=>'2');
$stmt = $mysqli->stmt_init();

$types = ''; $bindParam = array(); $where = ''; $count = 0;

// build the dynamic sql and param bind conditions
foreach($param as $key=>$val)
{
    $types .= 'i';
    $bindParam[] = '$p'.$count.'=$param["'.$key.'"]'; 
    $where .= "$key = ? AND ";
    $count++;
}

// prepare the query -- SELECT * FROM t1 WHERE customer_id = ? AND qty = ?
$sql = "SELECT * FROM t1 WHERE ".substr($where, 0, strlen($where)-4);
$stmt->prepare($sql);

// assemble the bind_param command
$command = '$stmt->bind_param($types, '.implode(', ', $bindParam).');';

// evaluate the command -- $stmt->bind_param($types,$p0=$param["customer_id"],$p1=$param["qty"]);
eval($command);

Is that last eval() statement a bad idea? I tried to avoid code injection by encapsulating values behind the variable name $param.

Does anyone have an opinion or other suggestions? Are there issues I need to be aware of?

How to&Answers:

I think it is dangerous to use eval() here.

Try this:

  • iterate the params array to build the SQL string with question marks "SELECT * FROM t1 WHERE p1 = ? AND p2 = ?"
  • call prepare() on that
  • use call_user_func_array() to make the call to bind_param(), passing in the dynamic params array.

The code:

call_user_func_array(array($stmt, 'bind_param'), array($types)+$param);

Answer:

I made a filter function which recives an array an asociative array like $_GET:

In model class I’ve defined a couple of properties including the schema:

private $table_name = "products";

protected $schema = [
    'id' => 'INT',
    'name' => 'STR',
    'description' => 'STR',
    'size' => 'STR',
    'cost' => 'INT',
    'active' => 'BOOL'
];

Then a filter method which recive an asociative arrays of conditions:

function filter($conditions)
{
    $vars   = array_keys($conditions);
    $values = array_values($conditions);

    $where = '';
    foreach($vars as $ix => $var){
        $where .= "$var = :$var AND ";
    }
    $where =trim(substr($where, 0, strrpos( $where, 'AND ')));

    $q  = "SELECT * FROM {$this->table_name} WHERE $where";
    $st = $this->conn->prepare($q);

    foreach($values as $ix => $val){
        $st->bindValue(":{$vars[$ix]}", $val, constant("PDO::PARAM_{$this->schema[$vars[$ix]]}"));
    }

    $st->execute();
    return $st->fetchAll(PDO::FETCH_ASSOC);
}

And works great to filter results

Answer:

You don’t really need prepared statements and bound arguments, because you can always use mysql_real_escape_string(). And you’re right; dynamically generated SQL is far more flexible and valuable.

Here’s a simple example using the regular mysql_* interface:

// Array of WHERE conditions
$conds = array("customer_id" => 1, "qty" => 2);

$wherec = array("1");
foreach ($conds as $col=>$val) $wherec[] = sprintf("`%s` = '%s'", $col, mysql_real_escape_string($val));

$result_set = mysql_query("SELECT * FROM t1 WHERE " . implode(" AND ", $wherec);

Of course, this is a simplistic example, and to make it useful you have to build and refine it a lot, but it shows the ideas and it’s really very very useful. For example, here is a completely generic function to insert a new row into an arbitrary table, with the columns filled with the values from an associative array and completely SQL-injection safe:

function insert($table, $record) {
    $cols = array();
    $vals = array();
    foreach (array_keys($record) as $col) $cols[] = sprintf("`%s`", $col);
    foreach (array_values($record) as $val) $vals[] = sprintf("'%s'", mysql_real_escape_string($val));

    mysql_query(sprintf("INSERT INTO `%s`(%s) VALUES(%s)", $table, implode(", ", $cols), implode(", ", $vals)));
}

// Use as follows:
insert("customer", array("customer_id" => 15, "qty" => 86));