Odd query behavior...

I have a question about the behavior of SELECT with multiple
tables. Take this example... 4 tables, 2 related by 1 many-to-many
table, and 1 unrelated table:

CREATE TABLE TGroup (
id SERIAL PRIMARY KEY,
name VARCHAR(16) UNIQUE
);

CREATE TABLE TUser (
id SERIAL PRIMARY KEY,
username VARCHAR(16) UNIQUE,
password VARCHAR(16)
);

CREATE TABLE TUnrelated (
id SERIAL PRIMARY KEY,
something VARCHAR(16) UNIQUE
);

CREATE TABLE TGroupTUserLink (
tgroup_id int4 NOT NULL,
tuser_id int4 NOT NULL,
UNIQUE (tgroup_id, tuser_id),
UNIQUE (tuser_id, tgroup_id),
FOREIGN KEY (tgroup_id) REFERENCES TGroup(id) ON DELETE CASCADE,
FOREIGN KEY (tuser_id) REFERENCES TUser(id) ON DELETE CASCADE
);

INSERT INTO TUser (username, password) VALUES ('bob', 'god');
INSERT INTO TUser (username, password) VALUES ('fred', 'sex');
INSERT INTO TGroup (name) VALUES ('user');
INSERT INTO TGroup (name) VALUES ('luser');

If I do a select from the TUser table, I get what is expected:

test=# select TUser.username from TUser;
username
----------
bob
fred
(2 rows)

However if I do a select from both the TUser and TGroup tables,
I get a product, even though I have expressed no joins:

test=# select TUser.username from TUser, TGroup;
username
----------
bob
bob
fred
fred
(4 rows)

And finally, if I do a select from the TUser and TUnrelated tables,
I get nothing, even though I have specified no joins:

test=# select TUser.username from TUser, TUnrelated;
username
----------
(0 rows)

What is going on here? Why am I getting "products"? It appears
to have something to do with how tables are related, but I don't
understand the full reasoning behind it. I have done futher tests
with a table that is related to a table that is related to yet
another table that I am querying, and I no longer get a product,
just the results I expect.

Any help would be appreciated...

Thanks.

Greg Brauer
greg(at)wildbrain(dot)com