Rows selection based on Column Values - oracle

Question

I have a problem where for a single value there can be maximum of two rows. I have to select only one row based on the column value.Below example can show my data and what I am trying to achieve.
Table Item with the following information
Item Dist_Building Country_Building
I123 B123 B245
I980 B980 B345
I780 B780 B445
Table item_info with the following columns
Item_number Building_Nbr Building_Area Value
I123 B123 District 10
I123 B245 Country 20
I980 B980 District 50
I780 B445 Country 20
Select the items from the Item table and check for the corresponding Building information present in Item_info table. If for a certain item, values are present at District and Country level then select the District level VALUE else select the Country Level VALUE(i.e. row with district area will take the priority over the Country )
Item_number Value
I123 10
I980 50
I780 20

Answer 1

are the Dist_Building, Country_Building, and Building_Nbr important here? It looks like theyre not, in which case if you create a new table from the item_info table like so...
select
item_number
, building_nbr
, sum(case when building_area = 'District' then value end) as District_Val
, sum(case when building_area = 'Country' then value end) as Country_Val
, coalesce(District_Val, Country_Val) as Value
into new_item_info
from item_info
group by
1,2
you should be able to just left join it to the item table.
(im not that familiar with Oracle SQL, so you may have to break the above code in to two steps)

Answer 2

One way to do this is with a join to either building, and then keep the one with the 'higher' area value (which works here as 'District' sorts after 'Country'):
select i.item,
max(ii.value) keep (dense_rank last order by ii.building_area) as value
from item i
join item_info ii on ii.item_number = i.item
and (
(ii.building_nbr = i.dist_building and ii.building_area = 'District')
or (ii.building_nbr = i.country_building and ii.building_area = 'Country')
)
group by i.item;
ITEM VALUE
---- ----------
I123 10
I780 20
I980 50
You could make that more explicit with a case expression in the order-by clause if you don't want to rely on that sort order.
You could also do two outer joins and use coalesce to pick the one you want:
select i.item,
coalesce(iid.value, iic.value) as value
from item i
left join item_info iid on iid.item_number = i.item
and iid.building_nbr = i.dist_building
and iid.building_area = 'District'
left join item_info iic on iic.item_number = i.item
and iic.building_nbr = i.country_building
and iic.building_area = 'Country';
ITEM VALUE
---- ----------
I123 10
I780 20
I980 50
db<>fiddle

Answer 3

You can achieve your goal by simply using sub-queries and the NVL function:
SELECT Item
, NVL((SELECT VALUE FROM item_info ii
WHERE ii.item_number = i.item
AND ii.Building_Nbr = i.Dist_Building)
,(SELECT VALUE FROM item_info ii
WHERE ii.item_number = i.item
AND ii.Building_Nbr = i.Country_Building)) VALUE
FROM Item i