what am I doing wrong with this query?

select array_to_string(conkey,',') from pg_constraint where contype =
'p' and conrelid = 17059

returns a value of 1,2,18 for the array to string function,

when I do this it does not return true:

select case when 18 in (array_to_string(conkey,',')) then true else
false end from pg_constraint where contype = 'p' and conrelid = 17059

but this one does return true

select case when 18 in (1,2,18) then true else false end from
pg_constraint where contype = 'p' and conrelid = 17059

How come the function does not work in the IN statement? I tried
casting it to a varchar, but that did not work either.

Thanks,

Tony