conditional join and conditional "cte"

i have this code

with cte1 as (

select ...... from TABLE 1......
where ......
)

with cte2 as 
( 
select .... from table2....
where.....
)

select cte1.id,cte2.name,cte2.last from cte1 join cte2 on cte1.id=cte2.id

what i want is get bool var from stored procedure and when it true i done want to do the join,and dont want that cte2 will be calculated(save a time)

something like:

if(@BollVar=false)
select cte1.id,cte2.name,cte2.last from cte1 join cte2 on cte1.id=cte2.id
else
select cte1.id,cte1.cat from cte1 
(and here i done want the cte2 even be calculated , because i donw need that)

i know that this if is not valid on sql but i just want to show what i want

how can i do this please?

thanks