Inserting data by user id - mysql

Question

I'm new to using database/sql so any links and education reads would be nice.
Right now it's inserting into the use column, which I understand because there no in id to the query, how can I make it so?
public function update_balance($balance){
$query = $this->db->prepare("INSERT INTO `users` (`balance`) VALUES (?)");
$query->bindValue(1, $balance);
try{
$query->execute();
} catch(PDOException $e) {
die($e->getMessage());
}
}
}
Here's me calling the function
$user = $users->userdata($_SESSION['id']);
$balance = $user['balance'];
$users->update_balance($balance);
I'm connecting via pdo if that makes any differencehttp://stackoverflow.com/editing-help
The table
ID | Username | Balance
1 | Something | 400
This site was really useful to figure it out
http://wiki.hashphp.org/PDO_Tutorial_for_MySQL_Developers
$query = $this->db->prepare("UPDATE users set balance = ? WHERE id = ?");
$query->bindValue(1, $balance);
$query->bindValue(2, $id);

Answer 1

you add a WHERE clause to tell it which row to insert into or to update.
what you are saying above is always add a new record with just this balance in it.
maybe something like this:
"INSERT INTO `users` ('id',`balance`) VALUES (?,?)"
and
$query->bindValue($user, $balance);