Question

I have a table with the following structure: ID, Month, Year, Value with values for one entry per id per month, most months have the same value.

I would like to create a view for that table that collapses the same values like this: ID, Start Month, End Month, Start Year, End Year, Value, with one row per ID per value.

The catch is that if a value changes and then goes back to the original, it should have two rows in the table

So:

• 100 1 2008 80
• 100 2 2008 80
• 100 3 2008 90
• 100 4 2008 80

should produce

• 100 1 2008 2 2008 80
• 100 3 2008 3 2008 90
• 100 4 2008 4 2008 80

The following query works for everything besides this special case, when the value returns to the original.

select distinct id, min(month) keep (dense_rank first order by month) 
over (partition   by id, value) startMonth, 
max(month) keep (dense_rank first order by month desc) over (partition
by id, value) endMonth, 
value

Database is Oracle

Answer 1

I got it to work as follows. It is heavy on analytic functions and is Oracle specific.

select distinct id, value,
decode(startMonth, null,
  lag(startMonth) over(partition by id, value order by startMonth, endMonth),  --if start is null, it's an end so take from the row before
startMonth) startMonth,

  decode(endMonth, null,
  lead(endMonth) over(partition by id, value order by startMonth, endMonth),  --if end is null, it's an start so take from the row after
endMonth) endMonth    

from (
select id, value, startMonth, endMonth from(
select id, value, 
decode(month+1, lead(month) over(partition by id,value order by month), null, month)     
startMonth, --get the beginning month for each interval
decode(month-1, lag(month) over(partition by id,value order by month), null, month)     
endMonth --get the end month for each interval from Tbl
) a 
where startMonth is not null or endMonth is not null --remain with start and ends only
)b

It might be possible to simplify some of the inner queries somewhat

The inner query checks if the month is a first/last month of the interval as follows: if the month + 1 == the next month (lag) for that grouping, then since there is a next month, this month is obviously not the end month. Otherwise, it is the last month of the interval. The same concept is used to check for the first month.

The outer query first filters out all rows that are not either start or end months (where startMonth is not null or endMonth is not null).
Then, each row is either a start month or an end month (or both), determined by whether start or end is not null). If the month is a start month, get the corresponding end month by getting the next (lead) endMonth for that id,value ordered by endMonth, and if it is an endMonth get the startMonth by looking for the previous startMonth (lag)