So, if $username is not populated (or otherwise false), but password is, you'll get:

$SQL = "SELECT FROM table WHERE  AND password='pwd'
[download]

Just join your WHERE clauses together using the string, ' AND ' as the separator.

my @WHERE = ();
push @WHERE, "username = '$username'" if $username;
push @WHERE, "password = '$password'" if $password;
push @WHERE, "name = '$name'" if $name;

$SQL = "SELECT FROM table WHERE " . join ' AND ', @WHERE;
[download]

Or see my post at 'Re: (2): mysql search statement'.

dmm

You can give a man a fish and feed him for a day ...
Or, you can teach him to fish and feed him for a lifetime