Select changed records - sql

Question

I have what I would have thought was a common problem but I have searched and cannot find an answer.
I have a large table containing (simplified) Month, Project, Type, Value
Jan ProjectA X 15000,
Jan ProjectB X 2000,
Jan ProjectB Y 2000,
Jan ProjectC X 3000,
.....
Feb ProjectA X 15000,
Feb ProjectB Y 4000,
Feb ProjectC X 3000,
Feb ProjectD X 8989,
.....
Mar ProjectA X 15000,
Mar ProjectB Y 4000,
Mar ProjectC X 3000,
Mar ProjectD X 8989,
.....
I need a query which shows:
All projects+type in Jan but not Feb AND
All projects+type in Feb but not Jan AND
All projects+type in both but with different Values
I need it between any two months sorry for not being clear
Any help would be greatly appreciated (I have tried sub queries with NOT IN and joins but I feel that I am missing something simple as my queries became very large and still did not work)

Answer 1

You should be able to do this with aggregation:
select project, type,
(case when sum(case when month = 'jan' then 1 else 0 end) > 0 and
sum(case when month = 'feb' then 1 else 0 end) = 0
then 'Jan-Only'
when sum(case when month = 'jan' then 1 else 0 end) = 0 and
sum(case when month = 'feb' then 1 else 0 end) > 0
then 'Feb-Only'
when max(case when month = 'jan' then value else 0 end) <>
max(case when month = 'feb' then value else 0 end)
then 'Different'
end) as Which
from largetable lt
group by project, type;
Probably the easiest way to filter out the ones where the values are equal in jan and feb is to use a subquery:
select t.*
from (select project, type,
(case when sum(case when month = 'jan' then 1 else 0 end) > 0 and
sum(case when month = 'feb' then 1 else 0 end) = 0
then 'Jan-Only'
when sum(case when month = 'jan' then 1 else 0 end) = 0 and
sum(case when month = 'feb' then 1 else 0 end) > 0
then 'Feb-Only'
when max(case when month = 'jan' then value else 0 end) <>
max(case when month = 'feb' then value else 0 end)
then 'Different'
end) as Which
from largetable lt
where month in ('jan', 'feb')
group by project, type
) t
where Which is not null;