帮我一下，我如何让下面的语句再简单呢？b和d是同一个表！
select a.*,b.description,d.description from
openquery(safelinked,'select * from asco.time_period') a,
openquery(safelinked,'select * from asco.days_lookup') b ,
openquery(safelinked,'select * from asco.days_lookup') d
where b.id=a.id
and d.id=a.id
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select * from
openquery(safelinked,'select a.*,b.description,d.description from asco.time_period a,asco.days_lookup b,asco.days_lookup d
where b.days_id=a.start_day and d.days_id=a.stop_day') x
order by tp_id