Return select statement with sql case statement

Hi,
I need to use conditional expression in my query, So I want to make a query
like this:
select numberOfPremiumDays
case when numberOfPremiumDays = date_part('day',
('2018-11-05'::timestamp) - ('2018-11-01'::timestamp)) then
select product_id,
price
from product
where occupation_type_id = 1
group by product_id, occupation_type_id

However, in the documentation I found that the return of case was a value,
not like in my case I want to return a select statement.
How can I use a conditional expression in a sql query?
Best Regards.