Count occurence of specific code per customer in 6 month period - sql

Question

I have a table that contains the following:
customerid | date (dmy) | productid
John | 1-3-14 | A
John | 7-5-14 | Y
John | 8-5-14 | Y
John | 1-10-15 | B
John | 1-11-15 | Y
Pete | 1-7-15 | Y
I need to find out how often customer X has bought Product Y in a six-month period.
The start of a period is defined as the first time a customer has bought one of the products A,B, C or Y. The endtime of a period is exactly six months after that.
The next period starts when the customer buys again one of the products A,B,C or Y.
So the output should be
customerid | period-start | period-end | countofY
John | 1-3-14 | 8-5-14 | 2
John | 1-10-15 | 1-11-15 | 1
Pete | 1-7-15 | 1-7-15 | 1

Answer 1

SELECT c.Customerid, MIN(c.pdate) AS startperiod, c1.endperiod,
(
SELECT COUNT(temp.productid) FROM Customer temp
WHERE temp.Customerid = c.Customerid
AND temp.pdate >= MIN(c.pdate)
AND temp.pdate <= c1.endperiod
GROUP BY temp.productid HAVING temp.productid ='Y'
)AS countOfY
FROM Customer c
CROSS APPLY
(
SELECT TOP 1 c1.pdate AS endperiod
FROM Customer c1
WHERE c1.Customerid = c.Customerid
AND c1.pdate >= c.pdate
AND
(
DATEDIFF(MONTH, c.pdate, c1.pdate) < 6
OR
(
SELECT TOP 1 t.pdate FROM Customer t
WHERE t.Customerid = c.Customerid
AND t.pdate < c1.pdate
) IS NULL
)
ORDER BY c1.pdate DESC
)AS c1 GROUP BY c1.endperiod, c.Customerid

Answer 2

;WITH CTE_DateRanges AS (
SELECT
customerid,
productid,
MIN(purchase_date) AS period_start,
DATEADD(MM, 6, MIN(purchase_date)) AS period_end
FROM
My_Table
GROUP BY
customerid,
productid
)
SELECT
DR.customerid,
DR.productid,
DR.period_start,
DR.period_end,
COUNT(*)
FROM
CTE_DateRanges DR
INNER JOIN My_Table MT ON
MT.customerid = DR.customerid AND
MT.productid = DR.productid AND
MT.purchase_date BETWEEN DR.period_start AND DR.period_end
GROUP BY
DR.customerid,
DR.productid,
DR.period_start,
DR.period_end,