Calculate duration of time ranges without overlap in PostgreSQL - postgresql

Question

I'm on Postgres 13 and have a table like this
| key | from | to
-------------------------------------------
| A | 2022-11-27T08:00 | 2022-11-27T09:00
| B | 2022-11-27T09:00 | 2022-11-27T10:00
| C | 2022-11-27T08:30 | 2022-11-27T10:30
I want to calculate the duration of each record, but without overlaps. So the desired result would be
| key | from | to | duration
----------------------------------------------------------
| A | 2022-11-27T08:00 | 2022-11-27T09:00 | '1 hour'
| B | 2022-11-27T09:00 | 2022-11-27T09:45 | '45 minutes'
| C | 2022-11-27T08:30 | 2022-11-27T10:00 | '15 minutes'
I guess, I need a subquery and subtract the overlap somehow, but how would I factor in multiple overlaps? In the example above C overlaps A and B, so I must subtract 30 minutes from A and then 45 minute from B... But I'm stuck here:
SELECT key, (("to" - "from")::interval - s.overlap) as duration
FROM time_entries, (
SELECT (???) as overlap
) s

Answer 1

select
key,
fromDT,
toDT,
(toDT-fromDT)::interval -
COALESCE((SELECT SUM(LEAST(te2.toDT,te1.toDT)-GREATEST(te2.fromDT,te1.fromDT))::interval
FROM time_entries te2
WHERE (te2.fromDT<te1.toDT or te2.toDT>te1.fromDT)
AND te2.key<te1.key),'0 minutes') as duration
from time_entries te1;
output:
key
fromdt
todt
duration
A
2022-11-27 08:00:00
2022-11-27 09:00:00
01:00:00
B
2022-11-27 09:00:00
2022-11-27 10:00:00
01:00:00
C
2022-11-27 08:30:00
2022-11-27 10:30:00
00:30:00
I renamed the columns from and to to fromDT and toDT to avoid using reserved words.
a, step by step, explanation is in the DBFIDDLE

Answer 2

Another approach.
WITH DATA AS
(SELECT KEY,
FROMDT,
TODT,
MIN(FROMDT) OVER(PARTITION BY FROMDT::DATE
ORDER BY KEY) AS START_DATE,
MAX(TODT) OVER(PARTITION BY FROMDT::DATE
ORDER BY KEY) AS END_DATE
FROM TIME_ENTRIES
ORDER BY KEY) ,STAGING_DATA AS
(SELECT KEY,
FROMDT,
TODT,
COALESCE(LAG(START_DATE) OVER (PARTITION BY FROMDT::DATE
ORDER BY KEY),FROMDT) AS T1_DATE,
COALESCE(LAG(END_DATE) OVER (PARTITION BY FROMDT::DATE
ORDER BY KEY),TODT) AS T2_DATE
FROM DATA)
SELECT KEY,
FROMDT,
TODT,
CASE
WHEN FROMDT = T1_DATE
AND TODT = T2_DATE THEN (TODT - FROMDT) ::Interval
WHEN T2_DATE < TODT THEN (TODT - T2_DATE)::Interval
ELSE (T2_DATE - TODT)::interval
END
FROM STAGING_DATA;