SQL Server 多表联合更新方法

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