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with tmp as
(select 'a' as col1,'t' as col2, null as col3 from dual
union all
select 'a' as col1,null as col2, 's' as col3 from dual
union all
select 'b' as col1,'t' as col2, null as col3 from dual
union all
select 'b' as col1,null as col2, 's' as col3 from dual
)
select col1,
max(col2) col2,
max(col3) col3
from tmp
group by col1