what am I doing wrong with this query?

[Tony Caduto <tony_caduto@xxxxxxxxxxxxxxxxxxxx>]

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