Distinct counts based on dates in SQL Server - sql

Question

I'm attempting to create a new table based on the following table:
SubjectNumber TestDates
001 11/12/12
001 01/10/15
001 04/03/13
002 05/21/14
003 08/06/15
002 09/12/18
002 03/30/12
003 09/07/18
004 10/14/11
005 02/05/14
005 02/06/14
I need a new table that will include the following:
1) Subject number
2) Their first test date
3) Their last test date
4) A count of the total number of tests
5) A column with 0's and 1's indicating whether or not the subject had any two test dates that were at least 30 days apart
The new table should look like the following:
SubjectNumber FirstTestDate LastTestDate TestCount ThirtyDaysApart
001 11/12/12 01/10/15 3 1
002 03/30/12 09/12/18 3 1
003 08/06/15 09/07/18 2 1
004 10/14/11 1 0
005 02/05/14 02/06/14 2 0
I'm using SQL Server 2017.
I have a temporary table called #Temp1 that I'd like to store the data in. This table is called #Temp.
Insert into #Temp1
SELECT SubjectNumber, WHERE
CASE MIN(TestDates) then FirstTestDate = TestDates
END
CASE MAX(TestDates) then LastTestDate = TestDates
END
FROM #Temp;

Answer 1

You can use lag() and conditional aggregation:
select subjectnumber, min(testdate), max(testdate),
max(case when prev_testdate < dateadd(day, -30, testdate) then 1 else 0 end) as diff30
from (select t.*,
lag(testdate) over (partition by subjectnumber order by testdate) as prev_testdate
from t
) t
group by subjectnumber;

Answer 2

You can try using lag() function
select subjectnumber,min(TestDates),max(TestDates),count(TestDates),
case when count(case when pdatediff=30 then 1 end)>=2 then 1 else 0 end as ThirtyDaysApart
from
(
select subjectnumber,TestDates,COALESCE (DATEDIFF(DAY,
LAG(TestDates) OVER (PARTITION BY subjectnumber
ORDER BY TestDates), TestDates) ,0) as pdatediff
from tablenmae
)X group by subjectnumber

Answer 3

The only tricky part is checking if two dates within a group are 30 days apart. Note that the following query returns 1 if any two dates, not necessarily consecutive, are 30 days apart:
WITH cte AS (
SELECT SubjectNumber, MIN(TestDates) FirstTestDate, MAX(TestDates) LastTestDate, COUNT(TestDates) TestCount
FROM #yourdata
GROUP BY SubjectNumber
)
SELECT *
FROM cte AS t
CROSS APPLY (
SELECT CASE WHEN COUNT(*) = 0 THEN 0 ELSE 1 END AS ThirtyDaysApart
FROM #yourdata AS o
INNER JOIN #yourdata AS n ON o.SubjectNumber = n.SubjectNumber AND n.TestDates >= DATEADD(DAY, 30, o.TestDates)
WHERE o.SubjectNumber = t.SubjectNumber
) AS CA
DB Fiddle