MySQL - Select row with column + X > column - mysql

Question

We have a database for patients that shows the details of their various visits to our office, such as their weight during that visit. I want to generate a report that returns the visit (a row from the table) based on the difference between the date of that visit and the patient's first visit being the largest value possible but not exceeding X number of days.
That's confusing, so let me try an example. Let's say I have the following table called patient_visits:
visit_id | created | patient_id | weight
---------+---------------------+------------+-------
1 | 2006-08-08 09:00:05 | 10 | 180
2 | 2006-08-15 09:01:03 | 10 | 178
3 | 2006-08-22 09:05:43 | 10 | 177
4 | 2006-08-29 08:54:38 | 10 | 176
5 | 2006-09-05 08:57:41 | 10 | 174
6 | 2006-09-12 09:02:15 | 10 | 173
In my query, if I were wanting to run this report for "30 days", I would want to return the row where visit_id = 5, because it's 28 days into the future, and the next row is 35 days into the future, which is too much.
I've tried a variety of things, such as joining the table to itself, or creating a subquery in the WHERE clause to try to return the max value of created WHERE it is equal to or less than created + 30 days, but I seem to be at a loss at this point. As a last resort, I can just pull all of the data into a PHP array and build some logic there, but I'd really rather not.
The bigger picture is this: The database has about 5,000 patients, each with any number of office visits. I want to build the report to tell me what the average wait loss has been for all patients combined when going from their first visit to X days out (that is, X days from each individual patient's first visit, not an arbitrary X-day period). I'm hoping that if I can get the above resolved, I'll be able to work the rest out.

Answer 1

You can get the date of the first and next visit using query like this (Note that this doesn't has correct syntax for date comparing and it is just an schema of the query):
select
first_visits.patient_id,
first_visits.date first_date,
max(next_visit.created) next_date
from (
select patient_id, min(created) as "date"
from patient_visits
group by patient_id
) as first_visits
inner join patient_visits next_visit
on (next_visit.patient_id = first_visits.patient_id
and next_visit.created between first_visits.created and first_visits.created + 30 days)
group by first_visits.patient_id, first_visits.date
So basically you need to find start date using grouping by patient_id and then join patient_visits and find max date that is within the 30 days window.
Then you can join the result to patient_visits to get start and end weights and calculate the loss.