forward CheckIfAccountExists(playerid, accountName[24]);
public CheckIfAccountExists(playerid, accountName[24])
{
    new query[128];
    mysql_format(db, query, sizeof(query), "SELECT uName FROM logindata WHERE uName = '%e';", accountName);
    mysql_function_query(db, query, false, "OnRequestNickRegistered", "i", playerid);
}

forward public OnRequestNickRegistered(playerid);
public OnRequestNickRegistered(playerid)
{
    new rows = cache_num_rows();
    new msg[128], name[128];
    if(rows)
    {
        format(msg, sizeof(msg), "Player %s exists in the database", playerName);
        SendClientMessage(playerid, COLOR_GREEN, msg);

        GetPlayerName(playerid, name, sizeof(name));
        format(msg, sizeof(msg), "Admin %s has just checked if the player %s exists in the database and he does.", name, playerName);
        print(msg);
    }
    else
    {
        format(msg, sizeof(msg), "Player %s doesn't exist in the database", playerName);
        SendClientMessage(playerid, COLOR_RED, msg);

        GetPlayerName(playerid, name, sizeof(name));
        format(msg, sizeof(msg), "Admin %s has just checked if the player %s exists in the database and he doesn't.", name, playerName);
        print(msg);
    }
    return 1;
}