Question : DateTime Group By question

I am writing a report and the users and they would like to select the grouping of the report.

All the grouping will be on a single datetime field (oddly enough named datetime).

They would like to be able to choose if the report is grouped by half hour, day, week, or month.

I can accomplish day by using:
GROUP BY CONVERT(VARCHAR(10),DateTime, 110)

But how can I accomplish half hour, week, or month?

Many thanks for the help.

Answer : DateTime Group By question

Group By DatePart(Week, [Datetime])

Group By DatePart(Month, [Datetime])

Group by Year([DateTime]), Month([DateTime]), Day([DateTime]), DatePart(Minute, [DateTime])/30