Checking to see if a MySQL row is populated - php

Question

I have a page that writes to a MySQL table. The table has a set amount of rows (24).
I have an $id variable that's set by a rand() function. I basically want to pull the row at that $id, so if $id was 3, I want to pull the third row. Then, I want to check if there is a price set at that row (indicating that the row is being used). If there is no price, I want to keep $id at the value it has been set at and proceed with the query. If there is a price, I want to re-randomize the $id variable, and check again if that row is used up. When it finds an empty row, proceed with the query.
My solution semi-works, but it seems to have a <10% chance of overwriting a used row, for some reason. I want it to never overwrite a used row.
Here's my code:
mysql_select_db("delives0_booklet", $con);
$query = "SELECT * FROM booklet WHERE id = '$id'";
$res = mysql_query($query,$con);
$newId = $id;
while($row = mysql_fetch_array($res))
{
if($row['price'] != 0)
{
do{
$newId = rand(1, 24);
}while($newId == $id);
}
}
$id = $newId;
mysql_query("UPDATE booklet SET price = '$price', advertiser = '$advertiser', image = '$image', monthsRemaining = '$monthsRemaining', availability = 1 WHERE id = '$id'");
Edit
I had the idea to do this. I loop through the table and I put the 'id' of each unfilled spot into an array. Then I pick randomly from that array. However, there seems to be a bug that I can't find, since the array keeps showing as having nothing in it, even after the loop is run, and $i is the correct figure.
mysql_select_db("delives0_booklet", $con);
$query = "SELECT * FROM booklet";
$res = mysql_query($query,$con);
$i = 0;
$isEmpty = array();
while($row = mysql_fetch_array($res))
{
if($row['price'] == 0)
{
$isEmpty[i] = $row['id'];
$i = $i + 1;
}
}
echo $i . " unfilled spots.";
$n = 0;
while($n<$i)
{
echo $isEmpty[$n];
$n = $n + 1;
}
if($i > 0)
{
$id = $isEmpty[rand(0, $i)];
}
if($i == 0)
{
echo 'All spots have been filled.';
}

Answer 1

I think it is a top level logic problem. Because you populate with random ids, you can get duplicate ids, and so when you update "WHERE id = '$id'" you may be picking up rows already populated.
I don't know your goal, but perhaps using an auto-increment id, and dropping rows that you want to get rid of, is the way to go. A rolling set of rows (24 at a time) but with ever increasing ids, would prevent mistaking one for the other.

Answer 2

If I understand the problem correct, this should work:
SELECT *
FROM booklet
WHERE price = 0 OR price IS NULL
ORDER BY RAND()