Negating the list of selected rows of a join

Hello,

I want to list the rows of a table with a text field whose values do not
exist in a similar field of another table. Basically what I want to get
is negated results of a join.

Lets say the tables table_a and table_b have the field name.

table_a table_b
name age name
----- --- -----
Peter 27 Paul
Paul 42
Mary 20

If I asked for a join like this:

SELECT table_a.name,table_a.age FROM table_a,table_b WHERE table_a.name=table_b.name

I would get:

name age
----- ---
Paul 42

But I want the opposite. I tried a non-equi join like this:

SELECT table_a.name,table_a.age FROM table_a,table_b WHERE table_a.name<>table_b.name

and I got:

name age
----- ---
Peter 27
Mary 20

It worked except for the case when table_b is empty. In this case the
nothing was returned. Is this the expected behaviour or is it a bug in
PostgreSQL?

How can I make a query that works the way I want all the time, even for the
case when table_b is empty?

Regards,
Manuel Lemos

E-mail: mlemos(at)acm(dot)org
URL: http://www.e-na.net/the_author.html
PGP key: finger://mlemos(at)zeus(dot)ci(dot)ua(dot)pt
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