Oracle SQL overlap between begin date and end date in 2 or more records - sql

Question

Database my_table:
id seq start_date end_date
1 1 01-01-2017 02-01-2017
1 2 07-01-2017 09-01-2017
1 3 11-01-2017 11-01-2017
2 1 20-01-2017 20-01-2017
3 1 01-02-2017 02-02-2017
3 2 03-02-2017 04-02-2017
3 3 08-01-2017 09-02-2017
3 4 09-01-2017 10-02-2017
3 5 10-01-2017 12-02-2017
My requirement is to get the first date (normally seq 1 start date) and end date (normally last seq end date) and the number of dates occurred during all seq for each unique ID.
Date occurred:
id 1 2 3
01-01-2017 20-01-2017 01-02-2017
02-01-2017 02-02-2017
07-01-2017 03-02-2017
08-01-2017 04-02-2017
09-01-2017 08-02-2017
11-01-2017 09-02-2017
10-02-2017
11-02-2017
12-02-2017
total 6 1 9
Here is the result I want:
id start_date end_date num_date
1 01-01-2017 11-01-2017 6
2 20-01-2017 20-01-2017 1
3 01-02-2017 12-02-2017 9
I have tried
SELECT id
, MIN(start_date)
, MAX(end_date)
, SUM(end_date - start_date + 1)
FROM my_table
GROUP BY id
and this SQL statement work fine in id 1 and 2 since there is no overlap date between begin date and end date. But for id 3, the result num_date is 11. Could you please suggest the SQL statement to solve this problem? Thank you.
One more question: The date in database is in datetime format. How do I convert it to date. I tried to use TRUNC function but it sometimes convert date to yesterday instead.

Answer 1

You need to count how many times an end_date equals the following start_date. For this you need to use the lag() or the lead() analytic function. You can use a case expression for the comparison, but alas you can't wrap the case expression within a COUNT or SUM in the same query; you need a subquery and an outer query.
Something like this; not tested, since you didn't provide CREATE TABLE and INSERT statements to recreate your sample data.
select id, min(start_date) as start_date, max(end_date) as end_date,
sum(end_date - start_date + 1 - flag) as num_days
from ( select id, start_date, end_date,
case when start_date = lag(end_date)
over (partition by id order by end_date) then 1
else 0 end as flag
from my_table
)
group by id;

Answer 2

SELECT id,
MIN( start_date ) AS start_date,
MAX( end_date ) AS end_date,
SUM( end_date - start_date + 1 ) AS num_days
FROM (
SELECT id,
GREATEST(
start_date,
COALESCE(
LAG( end_date ) OVER ( PARTITION BY id ORDER BY seq ) + 1,
start_date
)
) AS start_date,
end_date
FROM your_table
)
WHERE start_date <= end_date
GROUP BY id;