IN and subselects - Mailing list pgsql-sql

I am having problems getting IN in WHERE clauses to work together with
subselects.  That is, I want to do something like

     select * from a where a.id in (select id from b where b.n = 7);

The typical outcome is that no rows are selected even though some
match the criterion.  Doing the equivalent EXISTS condition will often
work, but I cannot always get it to work.

Is there a known problem with using IN?

Thanks for your help.

Cheers,
Brook