Exclude results in SQL script

[Hamlet]

I am little worried about sql script that I can not solve and so I'm asking for your help. Here is my situation: I have 3 tables: a customer table, resource table and a session table.

In the session table, I have list of resources for some customers.

My goal is to display all the resources of a client who does not includes (thus excluding them) in the session table.

In my session table I have a column that is named IDRES that refers to resources that are contained in the resource table (idresso).

Here is what I tried to exclude myself but it's not data that includes in the session table. In my application I included this constraint:

select codebar, quantity, ... from ressource, session, ...
where ...
and a.ressource.idresso <> a.session.idres

this constraint does not remove the display resources contained in the session table to display. But if I replace <> by = it gives me a good one that has the same id. I see too how to solve this problem.

[opaper]

SELECT fields FROM ressource WHERE ressource.idresso NOT IN (SELECT idres FROM session)

You can even use left outer join as follows:

SELECT fields 
FROM ressource LEFT OUTER JOIN session 
ON (ressource.idresso = session.idres)
WHERE session.idres IS NULL;

[Hamlet]

Thank you for your help. It worked. One more thing, which is "the cleaner": outer join or simply not in? In any case I thank you for your help.

[opaper]

Outer Join is much cleaner, and probably more efficient. However it depends what you mean by "clean" (ie the version with sub-query is more readable for those unfamiliar), but more efficient chances.

[CodGuru]

More efficient for sure if using an index! The tuning guide of oracle (O'Reilly) explains this easily:

Remember that indexes can tell you what is in a table but not what is not in a table. All references to NOT, !=, and <> disable index usage.