MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table mytable (
  id  int,
  customer_id int,
  start_at   date, 
  end_at date
);

insert into mytable values
(44,      37,  '2019-03-21',  '2019-03-21'),
(17819,   37,  '2020-03-23',  '2020-03-23'),
(22302,   37,  '2020-04-24',  '2021-07-25'),
(42213,   37,  '2021-04-25',  '2023-04-26'),
(92013,   37,  '2023-04-26',  '2024-04-26');

Records: 5  Duplicates: 0  Warnings: 0


select t1.id, 
       t1.customer_id, 
       coalesce(least(t1.start_at, t2.start_at), t1.start_at) as start_at,
       coalesce(greatest(t1.end_at, t2.end_at), t1.end_at) as end_at,
       t2.id as to_del
  from mytable t1
  left join mytable t2 on t1.customer_id = t2.customer_id
                        and t2.start_at > t1.start_at
                        and t2.start_at < t1.end_at


id
customer_id
start_at
end_at
to_del


44
37
2019-03-21
2019-03-21
null

17819
37
2020-03-23
2020-03-23
null

22302
37
2020-04-24
2023-04-26
42213

42213
37
2021-04-25
2023-04-26
null

92013
37
2023-04-26
2024-04-26
null


with cte as (
  select t1.id, 
       t1.customer_id, 
       coalesce(least(t1.start_at, t2.start_at), t1.start_at) as start_at,
       coalesce(greatest(t1.end_at, t2.end_at), t1.end_at) as end_at,
       t2.id as to_del
  from mytable t1
  left join mytable t2 on t1.customer_id = t2.customer_id
                        and t2.start_at > t1.start_at -- start after t1 is started
                        and t2.start_at < t1.end_at -- start before t1 is finished
)
select customer_id, sum(DATEDIFF(end_at, start_at)) as nb_days
from cte
where id not in (select to_del from cte where to_del is not null)
group by customer_id


customer_id
nb_days


37
1463

id	customer_id	start_at	end_at	to_del
44	37	2019-03-21	2019-03-21	null
17819	37	2020-03-23	2020-03-23	null
22302	37	2020-04-24	2023-04-26	42213
42213	37	2021-04-25	2023-04-26	null
92013	37	2023-04-26	2024-04-26	null