select 
posts_tbl.post_id,
case deleted_by_id is not null then
  'Removed by ' || deluser.name
else
  posts_tbl.message
end as message,
user.name
from posts_tbl 
left join removed_tbl on posts_tbl.post_id = removed_tbl.post_id 
left join authors as deluser on removed_tbl.deleted_by_id = deluser.author_id 
left join authors user on user.author_id = posts_tbl.author_id