postgres complicated query - postgresql

Question

I wonder is it possible to make such query. The problem is that I have a table where are some numbers for date.
Lets say I have 3 columns: Date, Value, Good/Bad
I.e:
2014-03-03 100 Good
2014-03-03 15 Bad
2014-03-04 120 Good
2014-03-04 10 Bad
And I want to select and subtract Good-Bad:
2014-03-03 85
2014-03-04 110
Is it possible? I am thinking a lot and don't have an idea yet. It would be rather simple if I had Good and Bad values in seperate tables.

Answer 1

The trick is to join your table back to it self as shown below. myTable as A will read only the Good rows and myTable as B will read only the Bad rows. Those rows then get joined into a signle row based on date.
SQL Fiddle Demo
select
a.date
,a.count as Good_count
,b.count as bad_count
,a.count-b.count as diff_count
from myTable as a
inner join myTable as b
on a.date = b.date and b.type = 'Bad'
where a.type = 'Good'
Output returned:
DATE GOOD_COUNT BAD_COUNT DIFF_COUNT
March, 03 2014 00:00:00+0000 100 15 85
March, 04 2014 00:00:00+0000 120 10 110
Another aproach would be to use Group by instead of the inner join:
select
a.date
,sum(case when type = 'Good' then a.count else 0 end) as Good_count
,sum(case when type = 'Bad' then a.count else 0 end) as Bad_count
,sum(case when type = 'Good' then a.count else 0 end) -
sum(case when type = 'Bad' then a.count else 0 end) as Diff_count
from myTable as a
group by a.date
order by a.date
Both approaches produce the same result.