SELECT Earliest Date from Grouped Results in DATEDIFF - sql

Question

From this table T2, I need to select the earliest date from each group by ID where the Prog is 'YY' and use it in DATEDIFF with respect to EDate:
+----+-----------+-----------+------+
| ID | SDate | Edate | Prog |
+----+-----------+-----------+------+
| 1 | 4/12/2016 | 5/18/2016 | XX |
| 1 | 4/1/2016 | 4/4/2016 | YY |
| 1 | 5/23/2016 | 5/28/2016 | YY |
| 2 | 9/21/2016 | 9/26/2016 | XX |
| 2 | 8/7/2016 | 8/9/2016 | YY |
| 3 | 8/2/2015 | 8/12/2015 | YY |
| 3 | 4/12/2015 | 4/18/2015 | YY |
+----+-----------+-----------+------+
And then show it with the aggregate level in Table T1 as the Desired Output:
+----+------+-----+-----------+------+
| ID | Name | Age | SDate | Days |
+----+------+-----+-----------+------+
| 1 | A | 52 | 4/1/2016 | 3 |
| 2 | B | 11 | 8/7/2016 | 2 |
| 3 | C | 24 | 4/12/2015 | 6 |
+----+------+-----+-----------+------+
Attempt:
SELECT
T1.ID,
T1.Name,
T1.Age,
MIN(T2.SDate) AS [SDate],
--DATEDIFF(day,MIN(T2.SDate),T2.EDate) AS [Days]
FROM T1
INNER JOIN T2
ON T1.ID=T2.ID
WHERE T2.Prog='YY'
GROUP BY
T1.ID,
T1.Name,
T1.Age
I commented out the DATEDIFF function for Days since I am not sure how to formulate that. Obviously, something like DATEDIFF(day,SELECT MIN(SDate) FROM T2 WHERE Prog='YY','Another Date') won't work since I will get an overall MIN(SDate) which won't be partitioned by ID and I can't do SELECT ID,MIN(SDate) FROM T2 WHERE Prog='YY' GROUP BY ID in the inner subquery either since DATEDIFF will only accept a Date field.
So how do I extract MIN(SDate) and calculate the DATEDIFF for corresponding Edate, for each grouped ID in that case?

Answer 1

Use the min window function to get the min sdate for each id and use it for computing the date difference.
SELECT ID,NAME,Age,DATEDIFF(DD,SDate,EDate)
FROM (
SELECT
T1.ID,
T1.Name,
T1.Age,
MIN(CASE WHEN T2.PROG = 'YY' THEN T2.SDate END) OVER(PARTITION BY T2.ID) AS [SDate],
T2.EDate
FROM T1
INNER JOIN T2 ON T1.ID=T2.ID
) x

Answer 2

Use MIN as a window function:
SELECT T1.ID,
T1.Name,
T1.Age,
DATEDIFF(day,
MIN(T2.SDate) OVER PARTITION BY (T1.ID, T1.Name, T1.Age),
T2.EDate) AS [Days]
FROM T1
INNER JOIN T2
ON T1.ID = T2.ID
WHERE T2.Prog = 'YY'