Generate a random value that doesn't exist in the same column

I already googled it, but I just found similar answers.
I'm not a back-end developer, so I don't know a lot of SQL. I need to generate and insert a 6 digit random value that involves numbers and characters (varchar, I think) that doesn't exist in the same column.

I tried this FLOOR(RAND() * 401) + 100 that I found somewhere, but it just generate numbers and... nothing more.

I'm lost.

Please, masters of databases, light my way with your random numbers ;-;

Solution

SELECT LPAD(CONV(RAND() * POW(36, 6), 10, 36), 6, 0)

This will create a 6 character "random" alphanumeric value.

But:

You will need to check if the value already exists in your table.
The distribution is not equal, because rand() retuns a FLOAT which only has a precision of 23 bit, while POW(36, 6) needs something like 32 bit.

Update:

However - because you will need to check, if the value already exists, you better create the number in PHP.

$success = false;
while (!$success) {
    $rndInt = rand(0, pow(36, 6) - 1);
    $rndStr = base_convert ($rndInt, 10, 36);
    $rndStr = str_pad($rndStr , 6, "0", STR_PAD_LEFT);

    $query = "SELECT 1 FROM your_table WHERE your_column = {$rndStr} LIMIT 1";
    $db->query($query);
    if (!$db->fetchColumn()) { // value does not exist yet
        // insert new random value
        $query = "INSERT INTO your_table (your_column) VALUES ({$rndStr})";
        $db->query($query);
        $success = true; // will terminate the loop
    } else { // value already exists
        // do nothing - try again in the next loop
    }
}

You will need to ajust the code to what ever you use for MySQL communication.