Regarding select distinct ...query

Hello All,

I have a select query with select distinct....this query is giving
me error as "unable to identify ordering operator '<' for type
'unknown'"

when i used this query without distinct, it works fine.
What is actually happening?? Can anyone fed some light on
it...???

SELECT prod_chain.depot_sending as depot_sending,
prod_chain.depot_sending as description,
'' as part,
visual_sc.xpos as xpos,
visual_sc.ypos as ypos,
visual_sc.vlevel as depthlevel,
visual_sc.parent_hpos as hparent,
visual_sc.node_hpos as hnode,
visual_sc.visible as nodevisible,
visual_sc.nameshown as nameshown,
0 as parentindex
FROM prod_chain LEFT OUTER JOIN visual_sc
ON ( TRIM (trailing '' from prod_chain.depot_receiving ) =
TRIM( trailing '' from visual_sc.parent_depot ) ) and
( TRIM( trailing '' from prod_chain.depot_sending ) =
TRIM( trailing '' from visual_sc.child_depot ) ) WHERE
( ( TRIM( trailing '' from prod_chain.depot_receiving )
= '' ) AND
( TRIM( trailing '' from visual_sc.initiator_depot )
= '' ) AND
( TRIM( trailing '' from visual_sc.initiator_part ) = '' )
AND
( TRIM( trailing '' from visual_sc.user_id ) = '' ) AND
( TRIM( trailing '' from visual_sc.sc_type ) = 'B'
)AND
visual_sc.vlevel = 2 AND
visual_sc.parent_hpos = 0
)

Thanks and regards,
Prachi