Am I trying the impossible?

I created a minimal test case. We have 3 simple tables:

create table t1
( a integer, b integer, c integer );

create table t2
( a integer, b integer, c integer );

create table t3
( b integer, d integer );


And the following query:

select y.D, sum(x.c),
(select sum(z.C)
from t2 z
join t3 w on z.B = w.B
where z.A = x.A)
from t1 x
join t3 y on x.B = y.B
GROUP by y.D;

...gives me this error:

Invalid expression in the select list (not contained in either an
aggregate function or the GROUP BY clause)


In both Firebird 1.5 and 2.0. The problem is the WHERE clause in
sub-select, but I don't understand why. I mean, I think I know why,
but I don't understand why this limitation exists.

M.