高手帮忙看一下存储过程?

set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
GO
ALTER proc [dbo].[test1]
as
declare @thisID as bigint
BEGIN TRANSACTION
SELECT top 1 @thisID=id FROM tab1 ROWLOCK WHERE flag=0
update tab1 set flag=1 where id=@thisID
select * from tab1 where id=@thisID
COMMIT TRANSACTION

多个客户端同时访问数据库，本意是让一条记录只被一个客户端读取，但有时还会出现一条记录同时被两个或多个客户端读取？

怎样改进呢？

谢谢！！