Set operations & tables with different columns

Question

I have a problem. I have 3 tables - Countries, Locations & Departments. I want to output the country_id and country_name from the countries tables for countries that have no departments. Locations has country_id and location_id columns and departments has location_id and department_id columns. All is in Database 11g. I can produce a table which gives me the locations with departments or without departments as follows:
select location_id from locations intersect select location_id from departments;
This gives me a list of locations with departments. Conversely I can get a list of locations without departments as follows:
select location_id from locations minus select location_id from departments;
However, to combine this output with the countries table I need to produce country_id in the output. However, when I try to add country_id to my code from above as follows:
select country_id, location_id from locations intersect select to_char(null), location_id from departments;
I get no rows found. I understand why. There is no intersection between country_id in locations and the null output from departments (where there is no country_id column). Alternatively, if I try as follow:
select country_id, location_id from locations minus select to_char(null), location_id from departments;
I get rows with every country_id whether they have departments or not. Again, I understand why. I am essentially subtracting nothing (the to_char(null) from departments) from the country_id column in locations and getting the entire country_id column as output.
How can I use set operations to produce the country_id column? I also do not want to show the location_id problem.
This problem is repeated when I try for the final country_id, country_name final output. The countries table has these columns. However, the locations table does not have the country_name column. It only has the country_id column. So a query such as this one:
select country_id, country_name from countries intersect country_id, to_char(null) from locations;
presents me with the same problem as above. The country_id column intersects nicely, but the country_name file does not intersect as it does not exist in the locations table. Again, how can I use set operations to produce the country_id and country_name columns.

Answer 1

Hi,
the method I outlined for you above should be fine. It doesn't matter that there are one-to-many relationships which may cause repeated output rows; set operations (except for UNION ALL ) always return distinct rows.
Can you do a query on the countries table, that shows all countries?
Can you do a query involving all 3 tables inner-joined, that shows the countries that are related to locations and departments? (Repeated rows are okay.)
Then you can do a MINUS, to get only the countries that are not related to departments.
If you get stuck, post a little sample data (CREATE TABLE and INSERT statements), the results you want from tha data, your best attemptat a query, and a description of the problem (including the full error message, if any).