This should answer it :
DECLARE @a int, @b-2 int, @C int, @d int
Select @a = 1, @b-2 = @a + 1, @C = @b-2 + 1, @d = @C + @b-2 + @a
Select @a as a, @b-2 as b, @C as c, @d as d
a b c d
----------- ----------- ----------- -----------
1 2 3 6
(1 ligne(s) affectée(s))