I have the following query:

SELECT
HPP.Period,
HPP.Clik,
SUM (HPP.New_Adds) AS HPP_GA_Count,
SUM (HPP.HPP_New_Adds) AS HPP_Adds_Count,

CASE WHEN HPP_New_Adds = 0 THEN 0
ELSE round(sum(HPP.HPP_New_Adds) / cast(sum(HPP.New_Adds) AS FLOAT),4) END AS [HPP_%]

FROM dbo.tbl_HPP_Monthly AS HPP

GROUP BY
Period,
Clik,

CASE WHEN HPP_New_Adds = 0 THEN 0
ELSE round(sum(HPP.HPP_New_Adds) / cast(sum(HPP.New_Adds) AS FLOAT),4) END

When attempting to run I get this message:

Msg 144, Level 15, State 1, Line 17
Cannot use an aggregate or a subquery in an expression used for the group by list of a GROUP BY clause.

I'm trying to add a new column that calculates the percentage of two columns....here are the columns... the result should be 30.30%.

Period Clik HPP_GA_Count HPP_Adds_Count
10/1/2012 1029 33 10


Thank you in advance for any help I can get!

-O