Comparing two sets of data - sql

Question

Very sorry if this has been answered in some way. I have checked all over and can't figure it out.
I need to find a way in postgresql to compare data from week to week. All data exists in the same table, and has a Week number column. Data will not always completely overlap but I need to compare data within groups when they do.
Say these are the data sets:
Week 2
+--------+--------+------+---------+-------+
| group | num | color| ID | week #|
+--------+--------+------+---------+-------+
| a | 1 | red | a1red | 2 |
| a | 2 | blue | a2blue | 2 |
| b | 3 | blue | b3blue | 2 |
| c | 7 | black| c7black | 2 |
| d | 8 | black| d8black | 2 |
| d | 9 | red | d9red | 2 |
| d | 10 | gray | d10gray | 2 |
+--------+--------+------+---------+-------+
Week 3
+--------+--------+------+---------+-------+
| group | num | color| ID | week #|
+--------+--------+------+---------+-------+
| a | 1 | red | a1red | 3 |
| a | 2 | green| a2green | 3 |
| b | 3 | blue | b3blue | 3 |
| b | 5 | green| b5green | 3 |
| c | 7 | black| c7black | 3 |
| e | 11 | blue | d11blue | 3 |
| e | 12 | other| d12other| 3 |
| e | 14 | brown| d14brown| 3 |
+--------+--------+------+---------+-------+
Each row has an ID made out of the group, number, and color values.
I need the query to grab all groups from Week 3, then for any groups in Week 3 that exist in Week 2:
flag ID's within the group that have changed, like in group A.
flag if any ID's were added or removed to the group, like in group B.
One function that would be nice to have, but is not essential, would be to have Week 3 compare against Week 1 for groups that do not exist in Week 2.
I have thought about trying to divide the two weeks up and use intercept/except to get results but I can't quite wrap my head around how I might get this to work correctly. Any tips would be much appreciated.

Answer 1

For just two (known) weeks you can do something like this:
select coalesce(w1.group_nr, w2.group_nr) as group_nr,
coalesce(w1.num, w2.num) as num,
case
when w1.group_nr is null then 'missing in first week'
when w2.group_nr is null then 'missing in second week'
when (w1.color, w1.id) is distinct from (w2.color, w2.id) then 'data has changed'
else 'no change'
end as status,
case
when
w1.group_nr is not null
and w2.group_nr is not null
and w1.color is distinct from w2.color then 'color is different'
end as color_change,
case
when
w1.group_nr is not null
and w2.group_nr is not null
and w1.id is distinct from w2.id then 'id is different'
end as id_change
from (
select group_nr, num, color, id, hstore
from data
where week = 2
) as w1
full outer join (
select group_nr, num, color, id
from data
where week = 3
) w2 on (w1.group_nr, w1.num) = (w2.group_nr, w2.num)
Getting the attributes that have changed is a bit clumsy. If you can live with a textual representation, you could use the hstore extension to display the differences:
select coalesce(w1.group_nr, w2.group_nr) as group_nr,
coalesce(w1.num, w2.num) as num,
case
when w1.group_nr is null then 'missing in first week'
when w2.group_nr is null then 'missing in second week'
when (w1.color, w1.id) is distinct from (w2.color, w2.id) then 'data has changed'
else 'no change'
end as status,
w2.attributes - w1.attributes as changed_attributes
from (
select group_nr, num, color, id, hstore(data) - 'week'::text as attributes
from data
where week = 2
) as w1
full outer join (
select group_nr, num, color, id, hstore(data) - 'week'::text as attributes
from data
where week = 3
) w2 on (w1.group_nr, w1.num) = (w2.group_nr, w2.num);