conditonal selection of 'AND' and 'SELECT'

I am working on Oracle reports . One of the parameters that a User
inputs is
'report_type'. Now, is there a way to do the following within a SQL
query with no use of procedures or functions.

select blaa from foo where

something = something
// if report_type='some_data' then
and somethingelse = something else
//else do nothing


Also, IS there a way of doing something like the following

// if report_type='some_data' then

select blaa from foo where
something = something
// if report_type='some_data' then
and somethingelse = something else
//else do nothing

else
select blass1 from foo where
something = something
// if report_type='some_data' then
and somethingelse = something else
//else do nothing

ALSO,

how would one achieve the following

select bla from ( if report_type=something
then
select decode(bla,..)
else
select decode(boo,..)
.......... rest is the same for both the selects

1. see my answer on your second posting
2.

select decode(report_type, 'something', decode(bla...), decode(boo...)) from table...

3.

This doesn't make much sense. The second select (select blass1) would always run into your else-branch and therefore would "do" (i'd prefer return) nothing