Conditional Union

I was wondering if there is a way to have the result of a case statement be a t-sql statement within a union? If not, is there another way to accomplish a conditional union?
IE.
select a, b from table_a
union
select a,b from table_b
union
CASE
when id = 1 then select a,b from Table_c
else select a.b from table_d

UNION cluase is to used to combine two statements results...
You can use CASE statement in WHERE clause...may be you are looking that one...
SELECT *
FROM T
WHERE col1 = CASE WHEN col2 < col3
THEN col2
ELSE col3
END

I understand what the two statements (union and case) do. What i am trying to accomplish is have a conditional union statement that depending on certain criteria it would union one statement instead of another. If I cant do a conditional Union my next thought would be if-else. The entire statement is going to be used as a view. can if-else be used in view?

ex
create view view_a(id, col_a,col_b,col_c,col_d)
AS
if (id = 1)
BEGIN
select * from table_a
union
select * from table_b
union
select * from table_c
END
ELSE
BEGIN
select * from table_a
union
select * from table_b
union
select * from table_d
END

Check out the CREATE VIEW syntax in BOL - there are no parameters for views. Why not create a stored procedure?
To keep this sort of code clean, without having to go spell out every possible combination of queries, use dynamic SQL - see http://www.sommarskog.se/dynamic_sql.html for some caveats.

use stored procedure to handle if ..else logic

I agree with Moh_hassan20. But I would suggest that you use the if logic and just insert into a temp table for each section rather than repeat the logic for the other queries for each IF statement.
Like :
create table #blah (field1....)
insert into #blah
select * from table_a
union
select * from table_b



if (id = 1)
BEGIN
insert into #blah
select * from table_c
END
ELSE
BEGIN
insert into #blah
select * from table_d
END

select * from #blah