Group by with gap in date sequence ("gaps and islands") - sql

Question

I am trying to solve a "gaps and islands" by date issue I'm facing (kudos to Gordon Linoff helping me identify this issue). I want to group the below table by person, office and job while respecting order by person,from_date. consider the table below:
declare #temp table(person varchar(25),office varchar(25),job varchar(25),from_date date,to_date date)
insert into #temp values ('jon','ny','programmer','1/1/2020','1/3/2020');
insert into #temp values ('jon','ny','programmer','1/4/2020','1/5/2020');
insert into #temp values ('jon','dc','programmer','1/6/2020','1/7/2020');
insert into #temp values ('jon','ny','programmer','1/8/2020','1/9/2020');
insert into #temp values ('lou','ny','programmer','1/1/2020','1/3/2020');
insert into #temp values ('lou','ny','programmer','1/4/2020','1/5/2020');
insert into #temp values ('lou','dc','programmer','1/6/2020','1/7/2020');
insert into #temp values ('lou','ny','programmer','1/8/2020','1/9/2020');
the intended output is

Answer 1

This is a type of gaps-and-islands problem. If there are no gaps in the dates, the simplest solution is the difference of row numbers:
select person, office, job, min(from_date), max(to_date)
from (select t.*,
row_number() over (partition by person, office, job order by from_date) as seqnum,
row_number() over (partition by person, office order by from_date) as seqnum_2
from t
) t
group by person, office, job, (seqnum - seqnum_2)

Answer 2

This is a general solution:
WITH preceders_and_followers AS (
SELECT
b.person,
b.office,
b.job,
b.from_date,
b.to_date,
CASE
WHEN EXISTS (
SELECT
c.*
FROM
ora$ptt_tmp c
WHERE
b.person = c.person
AND b.office = c.office
AND b.job = c.job
AND ( b.from_date - 1 BETWEEN c.from_date AND c.to_date )
) THEN
1
END AS has_preceder,
CASE
WHEN EXISTS (
SELECT
c.*
FROM
ora$ptt_tmp c
WHERE
b.person = c.person
AND b.office = c.office
AND b.job = c.job
AND ( b.to_date + 1 BETWEEN c.from_date AND c.to_date )
) THEN
1
END AS has_follower
FROM
ora$ptt_tmp b
ORDER BY
1,
2,
3
)
SELECT DISTINCT
pf1.person,
pf1.office,
pf1.job,
pf1.from_date,
(
SELECT
MIN(pf2.to_date)
FROM
preceders_and_followers pf2
WHERE
pf1.person = pf2.person
AND pf1.office = pf2.office
AND pf1.job = pf2.job
AND pf2.to_date >= pf1.from_date
AND has_follower IS NULL
) to_date
FROM
preceders_and_followers pf1
WHERE
pf1.has_preceder IS NULL
ORDER BY
1,
4,
2,
3;