Why can't I perform an aggregate function on an expression containing an aggregate but I can do so by creating a new select statement around it?

Question

Why is it that in SQL Server I can\'t do this:

select  sum(count(id)) as \'count\'
from    table

But I can do

select sum(x.         


        

Answer 1

Microsoft SQL Server doesn’t support it.

You can get around this problem by using a Derived table:

select sum(x.count)
from
(
    select  count(id) as 'count'
    from    table   
) x

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On the other hand using the below code will give you an error message.

select  sum(count(id)) as 'count'
from    table

Cannot perform an aggregate function on an expression containing an aggregate or a subquery