postgres case statement with subquery - postgresql

Question

I have a subquery like this
with subquery as (select host from table_A where << some condition >>)
and in my main query, I am querying data from another table called table_B, and one of the columns is called destination_host. Now I need to check if the destination_host is in the list returned from my subquery, then I want to output TypeA in my select statement or else TypeB. My select statement looks something like
select name, place, destination_host
from table_B
where <<some condition>>
I want to output a fourth column that is based on a condition check, let's say we call this host_category and if the destination_host value exists in the subquery then I want to add value typeA or else typeB. Please can you help me understand how to write this. I understand that it is hard to provide guidance if you don't have actual data to work with.
I tried using case statements such as this one:
when (destination_host in (select host from subquery)) THEN 'typeA'
when (destination_host not in (select host from subquery)) THEN 'typeB'
end as host_category
but I don't think this is the way to solve this problem.

Answer 1

I would use EXISTS:
WITH subquery AS (...)
SELECT CASE WHEN EXISTS (SELECT 1 FROM subquery
WHERE subquery.host = table_b.destination_host)
THEN 'typeA'
ELSE 'typeB'
END
FROM table_b;
With queries like that, you have to take care of NULL values. If table_b.destination_host is NULL, the row will always show up as typeB, because NULL = NULL is not TRUE in SQL.