You are mixing up your aggregates and scalar values. You are trying to use a case expression to control the whole column. This doesn't work at all. It is awfully hard to figure out what you are trying to do here but here is my shot in the dark.
SELECT ForcastUnitsTimesOMSCost, SUM(CTCBOHAndWCSBOHTimesOMSCost/IsNUll(ForcastUnitsTimesOMSCost, 0.00)) as Calculation
FROM MyTable
Where (CONVERT(date, GETDATE()) BETWEEN EventStartDate AND EventEndDate)
Group By ForcastUnitsTimesOMSCost
having IsNUll(ForcastUnitsTimesOMSCost, 0.00) < 0
If that doesn't help you will need to provide us a few things:
1. Sample DDL in the form of CREATE TABLE statements
2. Sample data in the form of INSERT INTO statements
3. Expected results based on the sample data
Please take a few minutes and read the first article in my signature for best practices when posting questions.
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