uniqueidentifier是全局唯一标识符 (GUID)
NEWID()返回类型为uniqueidentifier
SQL SERVER联机帮助中的例子：
declare @myid uniqueidentifier
set @myid=newid()
伊图教程网[www.etoow.com]
http://www.etoow.com/html/2007-08/1187531718.html
print 'Value of @myid is '+cast(@myid as varchar(255))
用NEWID()随机选出N条记录
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