case inside having

perhaps a having clause like this:

having @param <> 1
or @param is null
or @sum(field3) > 0.1

Here is an example:

declare @param int; set @param = 1;
declare @test table(a char(1), x decimal(5,1) );
insert into @test values('1', 0.1)
;
select a
from @test
group by a
having @param = 1 or @param is null
or sum(x) > 0.1;
/* -------- Output: --------
a
----
1
*/