Hi,
Please check my query here:
SELECT DATEPART(YYYY,datetimeStamp)*100+DATEPART(WEEK,datetimeStamp) WeekId,
DATEADD(week, DATEDIFF(week, 0, datetimeStamp), 0) [Week Beginning],
Register, AVG(Sales) [AverageDailySales]
FROM sales
GROUP BY DATEPART(YYYY,datetimeStamp)*100+DATEPART(WEEK,datetimeStamp),
DATEADD(week, DATEDIFF(week, 0, datetimeStamp), 0),
Register
Let me know if you need adjustment with the query.
Looking forward for your reply.
Regards,
Susanto