Fill the gaps, event based - sql-server

Question

I am trying to calculate churn of customers based on activity they could have done, opposed to churn by date that is the normal thing. We have events that is connected to a specific host, in my example all events are hosted by Alice but it could be different hosts.
All the people that follow a specific event should be placed in a category (new, active, churned and resurrected).
New: First time a person follow an event from the specific host.
Active: Follow again (and last event from specific host was also followed).
Churned: Follower had a chance to follow but didn't.
Resurrected: Follower that has churned has started to follow a previously followed host.
declare #events table (event varchar(50), host varchar(50), date date)
declare #eventFollows table (event varchar(50), follower varchar(50))
insert into #events values ('e_1', 'Alice', GETDATE())
insert into #events values ('e_2', 'Alice', GETDATE())
insert into #events values ('e_3', 'Alice', GETDATE())
insert into #events values ('e_4', 'Alice', GETDATE())
insert into #events values ('e_5', 'Alice', GETDATE())
insert into #eventFollows values ('e_1', 'Bob') --new
insert into #eventFollows values ('e_2', 'Bob') --active
--Bob churned
insert into #eventFollows values ('e_4', 'Megan') --new
insert into #eventFollows values ('e_5', 'Bob') --resurrected
insert into #eventFollows values ('e_5', 'Megan') --active
select * from #events
select * from #eventFollows
The expected outcome should be something like this
select 'e_1', 1 as New, 0 as resurrected, 0 as active, 0 as churned --First time Bob follows Alice event
union all
select 'e_2', 0 as New, 0 as resurrected, 1 as active, 0 as churned --Bob follows the next event that Alice host (considered as Active)
union all
select 'e_3', 0 as New, 0 as resurrected, 0 as active, 1 as churned --Bob churns since he does not follow the next event
union all
select 'e_4', 1 as New, 0 as resurrected, 0 as active, 0 as churned --First time Megan follows Alice event
union all
select 'e_5', 0 as New, 1 as resurrected, 1 as active, 0 as churned --Second time (active) for Megan and Bob is resurrected
I started with a query of something like below, but the problem is that I don't get all the events that the followers did not follow (but could have followed).
select a.event, follower, date,
LAG (a.event,1) over (partition by a.host, ma.follower order by date) as lag,
LEAD (a.event,1) over (partition by a.host, ma.follower order by date) as lead,
LAG (a.event,1) over (partition by a.host order by date) as lagP,
LEAD (a.event,1) over (partition by a.host order by date) as leadP
from #events a left join #eventFollows ma on ma.event = a.event order by host, follower, date
Any ideas?

Answer 1

This may seem a bit of an indirect approach, but it's possible to detect islands by checking for gaps in the numbers:
;with nrsE as
(
select *, ROW_NUMBER() over (order by event) rnrE from #events
), nrs as
(
select f.*,host, rnrE, ROW_NUMBER() over (partition by f.follower, e.host order by f.event ) rnrF
from nrsE e
join #eventFollows f on f.event = e.event
), f as
(
select host, follower, min(rnrE) FirstE, max(rnrE) LastE, ROW_NUMBER() over (partition by follower, host order by rnrE - rnrF) SeqNr
from nrs
group by host, follower, rnrE - rnrF --difference between rnr-Event and rnr-Follower to detect gaps
), stat as --from the result above on there are several options. this example uses getting a 'status' and pivoting on it
(
select e.event, e.host, case when f.FirstE is null then 'No participants' when f.LastE = e.rnrE - 1 then 'Churned' when rnrE = f.FirstE then case when SeqNr = 1 then 'New' else 'Resurrected' end else 'Active' end Status
from nrsE e
left join f on e.rnrE between f.FirstE and f.LastE + 1 and e.host = f.host
)
select p.* from stat pivot(count(Status) for Status in ([New], [Resurrected], [Active], [Churned])) p
The last 2 steps could be simplified, but getting the 'Status' this way might be reusable for other scenarios

Answer 2

This matches your desired result
SELECT
X.event, X.host, X.date,
IsNew = SUM(CASE WHEN X.FirstFollowerEvent = X.event THEN 1 ELSE 0 END),
IsActive = SUM(CASE WHEN X.lagFollowerEvent = X.lagEvent THEN 1 ELSE 0 END),
IsChurned = SUM(CASE WHEN X.follower IS NULL THEN 1 ELSE 0 END),
IsResurrected = SUM(CASE WHEN X.lagFollowerEvent <> X.lagEvent AND X.FirstFollowerEvent IS NOT NULL THEN 1 ELSE 0 END)
FROM
(
select
a.event, a.host, ma.follower, a.date,
FIRST_VALUE(a.event) over (partition by a.host, ma.follower order by a.date, a.event) as FirstFollowerEvent,
LAG (a.event,1) over (partition by a.host, ma.follower order by a.date, a.event) as lagFollowerEvent,
LAG (a.event,1) over (partition by a.host order by a.date, a.event) as lagEvent
FROM
#events a
LEFT join
#eventFollows ma on a.event = ma.event
) X
GROUP BY
X.event, X.host, X.date
ORDER by
X.event, X.host, X.date