Conditional Where Clause

Hi all,

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 include a predicate in the select statement.

Basically I am trying to write something similar to the one below but possible only usinf one select statement.

if @queryString is null then

select query ,sum(frequency)

from qt

group by query
else
select query ,sum(frequency)

from qt

group by query
where query = @queryString.

Now is there a way to achieve this thing without using two separate select? The actual code I am trying to write is much bigger and I am trying to see if there is more compact way of expressing things.

Thanks
Aye.