Checking mySQL db for duplicate uid

I am trying to 开发者_JAVA百科implement a check in my PHP code, that checks if there is a duplicate uid in the database, and if so, to assign a new uid, and check again, but I am having trouble nailing the logic, here is what I have thus far,

function check($uid){
    $sql = mysql_query("SELECT * FROM table WHERE uid='$uid'");
    $pre = mysql_num_rows($sql);

    if($pre >= 1){
        return false;
    }else{
        return true;
    }
}

And then using that function I thought of using a while loop to continue looping through until it evaluates to true

$pre_check = check($uid);

while($pre_check == false){
    //having trouble figuring out what should go here
}

So basically, once I have a usable uid, write everything to the database, else keep generating new ones and checking them till it finds one that is not already in use.

It is probably really simple, but for some reason I am having trouble with it. Thanx in advance!

$uid = 100;  // pick some other value you want to start with or have stored based on the last successful insert.
while($pre_check == false){
    $pre_check = check(++$uid);
}

Of course ths is exactly what 'auto incrementing' primary keys are useful for. Are you aware of 'auto incrementing' primary keys in mysql?

EDIT
In light of your comment regarding maintaining someone else's code that uses the random function like that (ewwwww)... I would use the method I suggest above and store the last inserted id somewhere you can read it again for the next user. This will allow you to "fill-in-the-blanks" for the uids that are missing. So, if for example you have uids 1, 2, 5, 9, 40, 100... you can start with $uid = 1; Your while loop will return once you get to 3. Now you store the 3 and create the new record. Next time, you start with $uid = 3; and so on. Eventually you will have all numbers filled in.

It is also important to realize that you will need to do the inserts by either locking the tables for WRITES. You don't want to get into a race condition where two different users are given the same uid because they are both searching for an available uid at the same time.

Indeed the best is to use autoincrement ids, but if you don't have the choice, you can do a reccursive function like that:

function find_uid() {
  $new_uid = rand(1000000000, 9999999999);
  $sql = mysql_query("SELECT COUNT(*) AS 'nb' WHERE uid=".$new_uid.";");
  $row = mysql_fetch_assoc();
  $pre = $row['nb'];
  return ($pre >= 1 ? find_uid() : $new_uid);
}

COUNT(*) should be more performant because the count is made by MySQL and not php.

By the way, if you need a new uid shouldn't the condition be ($pre > 0) instead of ($pre > 1) ?