Subquery uses ungrouped column

Hello!

Why the following query:

SELECT (select msc_id
              from collectors
              where id = substring(fileid from -1)
       ) msc_id
       from ip_data_records
       group by substring(fileid from -1)

gives me:

ERROR:  subquery uses ungrouped column "ip_data_records.fileid" from outer query
LINE 3:         where id = substring(fileid from -1)

but  the following query:

SELECT (select msc_id
              from collectors
              where id = fileid
       ) msc_id
       from ip_data_records
       group by fileid is working ok?

-- 
Alex Ignatov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company