my $sql = qq{
if not exists
    (select 1 from members where username = ?)
insert into members
    (username, password, email, name, url, date)
values (?, ?, ?, ?, ?, ?)
};
my $sth = $dbh->prepare($sql);
$sth->execute($username, $username,
    $password, $email, $name, $url, $date)