放一个整形字段SessionID,然后
declare @SessionID integer
Select @sessionID= 随机数
update TableName set SessionID=@SessionID where KeyID in(select Top 20 KeyID from TableName where @sessionID=0)
//选出这些数据作为返回值
select * from TableName where SessionID=@SessionID
//进行你要的操作
//最后
update TableName set SessionID=0 where SessionID=@SessionID