Sql query to create a report

Question

I am trying to extract a report for some pois, unfortunately I don't know how to formulate the query properly. Some data samples together with the rules for the report are described below:
with my_tab as (select 1 id, 'ACME' supplier, 1 poi, 'A' attr1, 'B' attr2 from dual union all
select 2 id, 'TOMA' supplier, 2 poi, 'C' attr1, 'D' attr2 from dual union all
select 3 id, 'ACME' supplier, 3 poi, 'A' attr1, 'B' attr2 from dual union all
select 4 id, 'TOMA' supplier, 3 poi, 'C' attr1, 'D' attr2 from dual union all
select 5 id, 'ACME' supplier, 4 poi, null attr1, null attr2 from dual union all
select 6 id, 'TOMA' supplier, 4 poi, 'C' attr1, 'D' attr2 from dual union all
select 7 id, 'ACME' supplier, 5 poi, null attr1, 'B' attr2 from dual union all
select 8 id, 'TOMA' supplier, 5 poi, 'C' attr1, 'D' attr2 from dual union all
select 9 id, 'ACME' supplier, 6 poi, null attr1, null attr2 from dual union all
select 10 id, 'TOMA' supplier, 6 poi, null attr1, 'D' attr2 from dual)
rules for the resultset:
a) each distinct poi has to occur exactly once
b) if there is more than one record having the same poi id, the attributes from supplier ACME are preferred
c) if ACME is not able to provide an attribute, the supplier TOMA has to step in
The result should look like this:
poi | attr1 | att2
1 | A | B |
2 | C | D |
3 | A | B |
4 | C | D |
5 | C | B |
6 | null | D |
Edited by: user8914294 on Dec 21, 2009 11:58 AM
Edited by: user8914294 on Dec 21, 2009 11:59 AM

Answer 1

Hi,
Welcome to the forum!
Here's one way:
SELECT       poi
,       MAX (attr1) KEEP (DENSE_RANK FIRST ORDER BY CASE
                                                                   WHEN  attr1  IS NOT NULL     THEN 1
                                                               END
                              ,           CASE  supplier
                                        WHEN  'ACME'  THEN  1
                                        WHEN  'TOMA'  THEN  2
                                                               END
                  ) AS attr_1
,       MAX (attr2) KEEP (DENSE_RANK FIRST ORDER BY CASE
                                                                   WHEN  attr2  IS NOT NULL     THEN 1
                                                               END
                              ,           CASE  supplier
                                        WHEN  'ACME'  THEN  1
                                        WHEN  'TOMA'  THEN  2
                                                               END
                  ) AS attr_2
FROM       my_tab
GROUP BY  poi
ORDER BY  poi
;This does not rely on the fact that there are only two suppliers, or that the combination (poi, supplier) is unique, or that the order of preference for suppliers is the same as alphabetic order.
This is an example of a Top-N Query , where you want N items (N=1 in this case) from the top of a sorted list. The problem is, how can we sort the values so that the more desirable ones come at the top of the list?
There are two things that make one value more desirable than another:
(1) Non-NULL values are better than NULL. After that:
(2) Rows where supplier='ACME' are more desireable than rows where supplier='TOMA'.
As it happens, both of these require CASE expressions (or siomething similar, like NVL2) to sort.
Thanks for posting the sample data in such a useful format! That helps a lot.