Formatting Functions and Group By

Hi,
I'm porting a bunch of queries from MySQL to Postgres 7.4 and am having a problem with one
particular area.  For example, a query like this works in MySQL:

select to_char(myCol,'Mon YY')
from myTable
group by to_char(myCol,'MM YYYY')
order by to_char(myCol,'MM YYYY')

Postgres will give me an error saying that "to_char(myCol,'Mon YY')" must be in the 'group by'.  I
understand why that normally needs to be the case, but in theory, it would be ok here because they
are actually the same values (in the select and group by) but just formatted differently.  I just
want the query to be grouped and ordered by month and year, but to be formatted differently in the
output.  

Any ideas?

Thanks!!

    
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