sql help

hi all

is there any way of getting around this problem?

SELECT DISTINCT ON (clust.clust) clust.clust where
clust.clust_id = clust_sys.clust_key
and
clust_sys.sys_key = system.key_system
and
system.location_key = 61
order by system.name;

ERROR: SELECT DISTINCT ON expressions must match initial ORDER BY
expressions

would one create a view or perhaps a temp table, and then do a second query
on that.
although i would prefer to just have one SQL statement.
the system.name will always be unique. there will normally be a number of
systems belonging to the cluster in clust.clust.

thanx
duncan.