update Table1 set BidAgent=x.BidAgent from Table1 t1,(select c.Id,a.BidAgent from Table1 c left join Table2 b on c.PurchaseItemId=b.PurchaseItemId left join Table3 a on a.Id=b.ParentId where a.BidAgent is not null and c.BidAgent is null) x where t1.Id=x.Id
先将需要的更新数据和待更新的表id查出来放在一个表x上,再关联这个表x更新就好