How to handle aggregate function in Case statements when it involves date columns

I’m calculating invoiceDate vs currentDate then sum Value/Amount column grouping by Customer but its returning “invoiceDate is not contained in either an aggregate function or the GROUP BY clause”

select Customer     
        ,case
            when datediff(dd,InvoiceDate,getdate()) <=30 then sum(InvoiceBal1)
            else 0
        end as [Current]
        ,case
            when datediff(dd,InvoiceDate,getdate()) between 31 and 60 then sum(InvoiceBal1)
            else 0
        end as [30 Days]
from CusInvoice
group by Customer

Answer

You want conditional aggregation:

select Customer     
       sum(case when datediff(day, InvoiceDate, getdate()) <= 30
                then InvoiceBal1 else 0
           end) as balance_current
       sum(case when datediff(day, InvoiceDate, getdate()) between 31 and 60
                then InvoiceBal1 else 0
           end) as balance_30days
from CusInvoice
group by Customer