Trying to use a sum in a where clause and I can't get the syntax right

In the select I need to sum(Day1)  field (which is a decimal field)  as Day1Hrs so I can test the field for 0 in my where clause

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and Day1Hrs = 0

I don't have enough experience with SQL to get the syntax right or even know if I can to that.  The where clause doesn't recognize the Day1Hrs field.  

Any help is appreciated.