Question

So I have a Databases exam in the coming weeks, and as usual I decided to go over the past papers to see how the question might be. Everything went along fine until I came across this question.

Write a SQL construct for the following query: Which forests are found
in the same geographical region (fo_loc) and are owned by the same
company. The output should have the following structure: Larger Forest
name, Smaller Forest Name, and Company Name. Try to avoid printing the
same forest names pairs more than once.

This is the schema relevant to the question:

TABLE: FOREST

Fo_name character(10) P.K.
Fo_Size Integer
Fo_loc Character(10)
Fo_Comp Character(10))

With my limited knowledge of SQL I do not think this question can be solved declaratively, so I tried also PL/SQL (or rather plpgsql) and I was getting closer and closer to the solution, but it soon dawned on me that I was using many commands we did not cover in class not to mention the solution was getting far from what one would expect out of a 3 mark question.

And so I decided to write here… what is it that I’m missing? Any help would be greatly appreciated.

Thanks

Answer 1

Just a self join should work:

SELECT a.Fo_Name as 'Larger', 
       b.Fo_Name as 'Smaller',
       a.fo_Comp
FROM Forest a
INNER JOIN Forest b
   ON a.fo_Comp= b.fo_Comp
   AND a.fo_loc = b.fo_loc
   AND a.fo_name <> b.fo_name
WHERE a.fo_size > b.fo_size