How to find the number of occurences within a date range? - sql

Question

Let's say I have hospital visits in the table TestData
I would like to know which patients have had a second hospital visit within 7 days of their first hospital visit.
How would I code this in SQL?
I have patient_id as a TEXT
the date is date_visit is also TEXT and takes the format MM/DD/YYYY
patient_id
date_visit
A123B29133
07/12/2011
A123B29133
07/14/2011
A123B29133
07/20/2011
A123B29134
12/05/2016
In the above table patient A123B29133 fulfills the condition as they were seen on 07/14/2011 which is less that 7 days from 07/12/2011

Answer 1

You can use a subquery with exists:
with to_d(id, v_date) as (
select patient_id, substr(date_visit, 7, 4)||"-"||substr(date_visit, 1, 2)||"-"||substr(date_visit, 4, 2) from visits
)
select t2.id from (select t1.id, min(t1.v_date) d1 from to_d t1 group by t1.id) t2
where exists (select 1 from to_d t3 where t3.id = t2.id and t3.v_date != t2.d1 and t3.v_date <= date(t2.d1, '+7 days'))
id
A123B29133

Answer 2

Since your date column is not in YYYY-MM-DD which is the default value used by several sqlite date functions, the substr function was used to transform your date in this format. JulianDay was then used to convert your dates to an integer value which would ease the comparison of 7 days. The MIN window function was used to identify the first hospital visit date for that patient. The demo fiddle and samples show the query that was used to transform the data and the results before the final query which filters based on your requirements i.e. < 7 days. With this approach using window functions, you may also retrieve the visit_date and the number of days since the first visit date if desired.
You may read more about sqlite date functions here.
Query #1
SELECT
patient_id,
visit_date,
JulianDay(visit_date) -
MIN(JulianDay(visit_date)) OVER (PARTITION BY patient_id)
as num_of_days_since_first_visit
FROM
(
SELECT
*,
(
substr(date_visit,7) || '-' ||
substr(date_visit,0,3) || '-' ||
substr(date_visit,4,2)
) as visit_date
FROM
visits
) v;
patient_id
visit_date
num_of_days_since_first_visit
A123B29133
2011-07-12
0
A123B29133
2011-07-14
2
A123B29133
2011-07-20
8
A123B29134
2016-12-05
0
Query #2
The below is your desired query, which uses the previous query as a CTE and applies the filter for visits less than 7 days. num_of_days <> 0 is applied to remove entries where the first date is also the date of the record.
WITH num_of_days_since_first_visit AS (
SELECT
patient_id,
visit_date,
JulianDay(visit_date) - MIN(JulianDay(visit_date)) OVER (PARTITION BY patient_id) num_of_days
FROM
(
SELECT
*,
(
substr(date_visit,7) || '-' ||
substr(date_visit,0,3) || '-' ||
substr(date_visit,4,2)
) as visit_date
FROM
visits
) v
)
SELECT DISTINCT
patient_id
FROM
num_of_days_since_first_visit
WHERE
num_of_days <> 0 AND num_of_days < 7;
patient_id
A123B29133
View on DB Fiddle
Let me know if this works for you.

Answer 3

I would like to know which patients have had a second hospital visit within 7 days of their first hospital visit.
You can use lag(). The following gets all rows where this is true:
select t.*
from (select t.*,
lag(date_visit) over (partition by patient_id order by date_visit) as prev_date_visit
from t
) t
where prev_date_visit >= date(date_visit, '-7 day');
If you just want the patient_ids, you can use select distinct patient_id.