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.