SQL question - how to output using iterative date logic in SQL Server - sql

Question

I have the following sample table (provided with single ID for simplicity - need to perform the same logic across all IDs)
ID Visit_date
-----------------
ABC 8/7/2019
ABC 9/10/2019
ABC 9/12/2019
ABC 10/1/2019
ABC 10/1/2019
ABC 10/8/2019
ABC 10/15/2019
ABC 10/17/2019
ABC 10/24/2019
Here is what I need to get the sample output
Mark the first visit as 1 in the "new_visit" column
Compare the subsequent dates with the 1st date until it exceeds 21 days condition. Example Sep 10 is compared to Aug 7 and it doesn’t fall within 21 days of Aug 7, therefore this is considered as another new_visit, so mark new_visit as 1
Then we compare Sep 10 with the subsequent dates with 21 days criteria and mark all of them as follow_up of Sep 10 visit. Eg. Sep 12, Oct 1 are within 21 days of Sep 10; hence they are considered as follow up visits, so mark "follow_up" as 1
When the subsequent date exceeds 21 days criteria of the previous new visit (e.g. Oct 8 compared to Sep 10) then Oct 8 will be considered a new visit & mark "New_visit" as 1 and the subsequent dates will be compared against Oct 8
Sample Output :
Dates New_Visit Follow_up
-----------------------------
8/7/2019 1
9/10/2019 1
9/12/2019 1
10/1/2019 1
10/1/2019 1
10/8/2019 1
10/15/2019 1
10/17/2019 1
10/24/2019 1

Answer 1

You need a recursive query for this.
You would enumerate the rows, then walk through the dataset by ascending date, while keeping track of the first visit date of each group; when the interval since the last first visit exceeds 21 days, the date of the first visit resets, and a new group starts.
with
data as (
select t.*, row_number() over(partition by id order by date) rn
from mtytable t
),
cte as (
select id, visit_date, visit_date first_visit_date
from data
where rn = 1
union all
select c.id, d.visit_date, case when d.visit_date > datead(day, 21, c.first_visit_date) then d.visit_date else c.first_visit_date end
from cte c
inner join data d on d.id = c.id and d.rn = c.rn + 1
)
select
id,
date,
case when visit_date = first_visit_date then 1 else 0 end as is_new
case when visit_date = first_visit_date then 0 else 1 end as is_follow_up
from cte
If a patient may have more than 100 visits, then you need to add option (maxrecursion 0) at the very end of the query.

Answer 2

You need a recursive CTE to handle this. This is the idea, although the exact syntax might vary by database:
with recursive t as (
select id, date,
row_number() over (partition by id order by date) as seqnum
from yourtable
),
recursive cte as (
select id, date, visit_start as date, 1 as is_new_visit
from t
where id = 1
union all
select cte.id, t.date,
(case when t.date < visit_start + interval '21 day'
then cte.visit_start else t.date
end) as visit_start,
(case when t.date < cte.visit_start + interval '21 say'
then 0 else 1
end) as is_new_visit
from cte join
t
on t.id = cte.id and t.seqnum = cte.seqnum + 1
)
select *
from cte
where is_new_visit = 1;