Conditional Group By In T-SQL

I have a table QT defined as

CREATE TABLE [dbo].[QT](
[Query] [nvarchar](50) NULL,
[Frequency] [int] NULL
) ON [PRIMARY]

Now based on a parameter I want to return it's columns grouped by the query column or return all the rows or the rows grouped by query columns.

This is the sql I am trying to write


select

case when @groupbyQuery = 'true' then 'ALL' else query end

,sum(frequency)

from qt

group by query

But this does not give me the desired result. ?Is there a way to do conditional group by?
Any other way of achieving this?

Thanks
Aye.