sql中while遍历更新字段数据

while exists(select 1 from xlhhis..emrzy_doclist where docbill is null)
begin
update xlhhis..emrzy_doclist set docbill=[dbo].[fun_emr_docbill]('') where objid=
(select top 1 objid from xlhhis..emrzy_doclist  where docbill is null)
end