Intentional, or bug?

Hi!

I probably found an inconsistence of the working of the =
operator. Please read ahead, if you're interested.

I have two tables, let't call them a and b. They are:

a)      x   |  y
-----------------------------      1   |  100      2   |  NULL

b)      y   |  z
-----------------------------      1   |  'hallelujah'     NULL |  'hmm...'


Now let's issue this query:

select b.z from a,b where a.y=b.y;

The answer is: 'hallelujah', but it omits 'hmm...'.

Now let's issue the following:

select NULL = NULL;

The answer is: true!!!

My question is why does it omit the rows with NULL at the
first case?

However, I have a real life problem, when not all data is given in
a column, so I need a query to include the NULL=NULL rows. I can
workaround it as defining a new operator, ~=, with the
meaning "both null, or =". I did it, it works, but very slow.

So, is it an intentinal way of functioning, or it is bug somewhere?

Thanks,
Baldvin