counting distinct result of join

Hi,

are there an efficient way to count distinct result from join ?
I know it's possible in 7.0 to do 

select count(distinct m.msg_id) from messages m, message_section_map ms
where m.msg_id = ms.msg_id;

but I need it working in production for 6.5.3
I'd like not to use temp tables as I dont' have any experience
with them in production.
Regards,
    Oleg

_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83