Unpack expression results from case statement - postgresql

Question

Four categories in category table.
id | name
--------------
1 | 'wine'
2 | 'chocolate'
3 | 'autos'
4 | 'real estate'
Two of the many (thousands of) forecasters in forecaster table.
id | name
--------------
1 | 'sothebys'
2 | 'cramer'
Relevant forecasts by the forecasters for the categories in the forecast table.
| id | forecaster_id | category_id | forecast |
|----+---------------+-------------+--------------------------------------------------------------|
| 1 | 1 | 1 | 'bad weather, prices rise short-term' |
| 2 | 1 | 2 | 'cocoa bean surplus, prices drop' |
| 3 | 1 | 3 | 'we dont deal with autos - no idea' |
| 4 | 2 | 2 | 'sell, sell, sell' |
| 5 | 2 | 3 | 'demand for cocoa will skyrocket - prices up - buy, buy buy' |
I want prioritized mapping of (forecaster, category, forecast) such that, if a forecast exists for some primary forecaster (e.g. 'cramer') use it because I trust him more. If a forecast exists for some secondary forecaster (e.g. 'sothebys') use that. If no forecast exists for a category, return a row with that category and null for forecast.
I have something that almost works and after I get the logic down I hope to turn into parameterized query.
select
case when F1.category is not null
then (F1.forecaster, F1.category, F1.forecast)
when F2.category is not null
then (F2.forecaster, F2.category, F2.forecast)
else (null, C.category, null)
end
from
(
select
FR.name as forecaster,
C.id as cid,
C.category as category,
F.forecast
from
forecast F
inner join forecaster FR on (F.forecaster_id = FR.id)
inner join category C on (C.id = F.category_id)
where FR.name = 'cramer'
) F1
right join (
select
FR.name as forecaster,
C.id as cid,
C.category as category,
F.forecast
from
forecast F
inner join forecaster FR on (F.forecaster_id = FR.id)
inner join category C on (C.id = F.category_id)
where FR.name = 'sothebys'
) F2 on (F1.cid = F2.cid)
full outer join category C on (C.id = F2.cid);
This gives:
'(sothebys,wine,"bad weather, prices rise short-term")'
'(cramer,chocolate,"sell, sell, sell")'
'(cramer,autos,"demand for cocoa will skyrocket - prices up - buy, buy buy")'
'(,"real estate",)'
While that is the desired data it is a record of one column instead of three. The case was the only way I could find to achieve the ordering of cramer first sothebys next and there is lots of duplication. Is there a better way and how can the tuple like results be pulled back apart into columns?
Any suggestions, especially related to removal of duplication or general simplification appreciated.

Answer 1

This sounds like a case for DISTINCT ON (untested):
SELECT DISTINCT ON (c.id)
fr.name AS forecaster,
c.name AS category,
f.forecast
FROM forecast f
JOIN forecaster fr ON f.forecaster_id = fr.id
RIGHT JOIN category c ON f.category_id = c.id
ORDER BY
c.id,
CASE WHEN fr.name = 'cramer' THEN 0
WHEN fr.name = 'sothebys' THEN 1
ELSE 2
END;
For each category, the first row in the ordering will be picked. Since Cramer has a higher id than Sotheby's, it will be given preference.
Adapt the ORDER BY clause if you need a more complicated ranking.