Consolidate information (time serie) from two tables - sql

Question

MS SQL Server
I have two tables with different accounts from the same customer:
Table1:
ID
ACCOUNT
FROM
TO
1
A
01.10.2019
01.12.2019
1
A
01.02.2020
09.09.9999
and table2:
ID
ACCOUNT
FROM
TO
1
B
01.12.2019
01.01.2020
As result I want a table that summarize the story of this costumer and shows when he had an active account and when he doesn't.
Result:
ID
FROM
TO
ACTIV Y/N
1
01.10.2019
01.01.2020
Y
1
02.01.2020
31.01.2020
N
1
01.02.2020
09.09.9999
Y
Can someone help me with some ideas how to proceed?

Answer 1

This is the typical gaps and island problem, and it's not usually easy to solve.
You can achieve your goal using this query, I will explain it a little bit.
You can test on this db<>fiddle.
First of all... I have unified your two tables into one to simplify the query.
-- ##table1
select 1 as ID, 'A' as ACCOUNT, convert(date,'2019-10-01') as F, convert(date,'2019-12-01') as T into ##table1
union all
select 1 as ID, 'A' as ACCOUNT, convert(date,'2020-02-01') as F, convert(date,'9999-09-09') as T
-- ##table2
select 1 as ID, 'B' as ACCOUNT, convert(date,'2019-12-01') as F, convert(date,'2020-01-01') as T into ##table2
-- ##table3
select * into ##table3 from ##table1 union all select * from ##table2
You can then get your gaps and island using, for example, a query like this.
It combines recursive cte to generate a calendar (cte_cal) and lag and lead operations to get the previous/next record information to build the gaps.
with
cte_cal as (
select min(F) as D from ##table3
union all
select dateadd(day,1,D) from cte_cal where d < = '2021-01-01'
),
table4 as (
select t1.ID, t1.ACCOUNT, t1.F, isnull(t2.T, t1.T) as T, lag(t2.F, 1,null) over (order by t1.F) as SUP
from ##table3 t1
left join ##table3 t2
on t1.T=t2.F
)
select
ID,
case when T = D then F else D end as "FROM",
isnull(dateadd(day,-1,lead(D,1,null) over (order by D)),'9999-09-09') as "TO",
case when case when T = D then F else D end = F then 'Y' else 'N' end as "ACTIV Y/N"
from (
select *
from cte_cal c
cross apply (
select t.*
from table4 t
where t.SUP is null
and (
c.D = t or
c.D = dateadd(day,1,t.T)
)
) t
union all
select F, * from table4 where T = '9999-09-09'
) p
order by 1
option (maxrecursion 0)
Dates like '9999-09-09' must be treated like exceptions, otherwise I would have to create a calendar until that date, so the query would take long time to resolve.