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select distinct a.viewer_id id
from (
select article_id,author_id,viewer_id,view_date
from Views
group by article_id,author_id,viewer_id,view_date
order by view_date,article_id
)a
group by a.view_date,a.viewer_id
having count(distinct a.article_id)>=2
order by a.viewer_id
