Duplicate table in Oracle query - sql

Question

I have a query that looks like this:
SELECT *
FROM
employees e,
departments d1,
departments d2,
departments d3,
departments d4
WHERE e.dep1 = d1.dep(+)
AND e.dep2 = d2.dep(+)
AND e.dep3 = d3.dep(+)
AND e.dep4 = d4.dep(+);
Is there a better way to write this so that I don't need to use the same table in my query multiple times? I know Oracle's optimizer probably works around this rather nicely, but if there is a more efficient way to write a query like this, I'm all ears. Keep in mind that the above is merely an example, my actual query has a lot more going on in it. I'm using Oracle 11.2.0.3 on Windows 2003 x64.
Thanks,
Tom

Answer 1

This is perfectly valid.
The contrived example me and my colleague often use is city_of_birth and city_of_residence. Let's assume we want to query employees who now live in a different country to which they were born in (and that all countries have states).
You would query this as follows:
select e.*
from employees e,
cities city_of_birth,
cities city_of_residence,
states state_of_birth,
states state_of_residence
where e.city_of_birth_id = city_of_birth.id
and e.city_of_residence_id = city_of_residence.id
and city_of_birth.state_id = state_of_birth.id
and city_of_residence.state_id = state_of_residence.id
and state_of_birth.country_id != state_of_residence.country_id;
The thing to note is that tables need to be referenced in the from clause of a query as often as there are different meanings to them.
Another way to think about it: you need to reference the same table multiple times in the from clause if you're going to be selecting different rows from each "instance".

Answer 2

If you want the details to all the departments flat in one row you need several joins but I suggest to write it this way:
SELECT *
FROM employees e
LEFT JOIN departments d1 on ( d1.dep = e.dep1 )
LEFT JOIN departments d2 on ( d2.dep = e.dep1 )
LEFT JOIN departments d3 on ( d3.dep = e.dep1 )
LEFT JOIN departments d4 on ( d4.dep = e.dep1 )

Answer 3

this doesn't make much sense to me... you are comparing an employee to find their department (when they have many possible in non-normalized columns) - what about using an OR clause...
SELECT *
FROM
employees e,
departments d
WHERE ( e.dep1 = d.dep
OR e.dep2 = d.dep
OR e.dep3 = d.dep
OR e.dep4 = d.dep );
or better - rework your table so it is properly structured. :)