update ccm21.riversegments set distance_source=shape_leng where gid in(
select distinct on (rs.gid) rs.gid from ccm21.riversegments rs
where rs.wso_id in (select wso_id from europe.wso where area='Meuse')
except (        
select distinct on (rs.gid) rs.gid  from ccm21.riversegments rs
        inner join ccm21.riversegments rup on rup.tonode=rs.fromnode
        where rs.wso_id in (select wso_id from europe.wso where area='Meuse'))
        order by gid)