[RajatPawar]

Just use recursion

pawn Код:

stock RandomCarNumbers()
{
    new string[64], query[128];
    format(string,sizeof(string),"{000000}%c%c%c-%i%i%i",(65+random(26)),(65+random(26)),(65+random(26)),1+random(9),random(10),random(10));
    MySQLCheck();
    format(query, sizeof(query), "SELECT * FROM 'cars' WHERE numeriai = %s", string);
    mysql_query(query);
    mysql_store_result();
    if(mysql_num_rows() > 0)
    return
        RandomCarNumbers();
    else
    {
        mysql_free_result();
        return string;
    }
}

It will keep calling itself until you have a car number that's unique.