Eliminate duplicate values and replace them with null values oracle?

I have a table like

source  dest1  dest2
--------------------
su1     du1    st1
su1     du1    st2
su1     du1    st3
su2     du1    st1
su2     du2    st4
su2     du2    st1

But my desired table is

source  dest1  dest2
--------------------
su1     du1    st1
su1     du1    st2
su1     du1    st3
su2    (null) (null)
su2     du2    st4
su2     du2   (null)

Rule here is Replacing dest2 by null if it appears once and Replacing dest1, dest2 by null if dest1 appears once…

Answer

You seem to be null’ing out dest2 after it appears once, and the pair when both appear. Assuming that source provides the ordering, you can do:

select source,
       (case when row_number() over (partition by dest1, dest2 order by source) = 1
             then dest1
        end) as dest1,
       (case when row_number() over (partition by dest2 order by source) = 1
             then dest2
        end) as dest2
from t;