Creating ranged mapping records from yearly entries - sql

Question

I have a table that maps an ID to an Associated ID (AssocID) over time and the database is built having one record per year. I would like to roll up the table have one record for each period of association.
Current Example:
ID AssocID Start End
1 a 2000 2001
1 a 2001 2002
1 b 2002 2003
1 b 2003 2004
1 a 2004 2005
...
1 a 2017 2018
2 c 2000 2001
2 c 2001 2002
2 d 2002 2003
...
2 d 2017 2018
and I am trying to make it look more like this:
ID AssocID Start End
1 a 2000 2002
1 b 2002 2004
1 a 2004 2018
2 c 2000 2002
2 d 2002 2018
My main problem is that ID '1' goes back to AssocID 'a' after time and using DISTINCT (ID, AssocID) and MIN (Start) misses the second time ID '1' maps to AssocID 'a'
Any help appreciated :)

Answer 1

You can use this.
-- Sample Data
DECLARE #MyTable TABLE (ID INT, AssocID VARCHAR(10), Start INT, [End] INT)
INSERT INTO #MyTable VALUES
(1, 'a', 2000, 2001),
(1, 'a', 2001, 2002),
(1, 'b', 2002, 2003),
(1, 'b', 2003, 2004),
(1, 'a', 2004, 2005),
(1, 'a', 2017, 2018),
(2, 'c', 2000, 2001),
(2, 'c', 2001, 2002),
(2, 'd', 2002, 2003),
(2, 'd', 2017, 2018)
-- Query
SELECT ID, AssocID, MIN(Start) [Start], MAX([End]) [End] FROM
( SELECT *,
GRP = ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Start) - ROW_NUMBER() OVER(PARTITION BY ID, AssocID ORDER BY Start)
FROM #MyTable ) T
GROUP BY ID, AssocID, GRP
ORDER BY ID, [Start]
Result:
ID AssocID Start End
----------- ---------- ----------- -----------
1 a 2000 2002
1 b 2002 2004
1 a 2004 2018
2 c 2000 2002
2 d 2002 2018

Answer 2

This is an example of a gaps and islands problem. You need to first identify the start of each group grp_start and then group by each grp to find the min / max
declare #T table (ID int, AssocID varchar(3), Start int, [End] int)
insert into #T (ID, AssocID, Start, [End]) values
(1, 'a', 2000, 2001),(1, 'a', 2001, 2002),(1, 'b', 2002, 2003),(1, 'b', 2003, 2004),(1, 'a', 2004, 2005),(1, 'a', 2005, 2006),(1, 'a', 2006, 2007),(1, 'a', 2007, 2008),(1, 'a', 2008, 2009),(1, 'a', 2009, 2010),(1, 'a', 2010, 2011),(1, 'a', 2011, 2012),(1, 'a', 2012, 2013),(1, 'a', 2013, 2014),(1, 'a', 2014, 2015),(1, 'a', 2015, 2016),(1, 'a', 2016, 2017),(1, 'a', 2017, 2018),(2, 'c', 2000, 2001),(2, 'c', 2001, 2002),(2, 'd', 2002, 2003),(2, 'd', 2017, 2018)
select
ID,
AssocID,
min(Start),
max([End])
from
(
select *,
sum([grp_start]) over (partition by ID, AssocID order by [End]) as grp
from
(
select *,
case
when
lag([End]) over (partition by ID, AssocID order by [End]) <> [Start]
then 1 else 0
end as [grp_start]
from #T
) as T
)as T
group by ID, AssocID, grp
order by ID, min(Start), max([End])