SQL Server--> SELECT TOP 50 * FROM TAB_USERS ORDER BY NEWID();

　　ORACLE--> SELECT * FROM (SELECT * FROM TAB_USERS ORDER BY DBMS_RANDOM.RANDOM()) WHERE RONUM<=50

　　MYSQL--> SELECT * FROM TAB_USERS ORDER BY RAND() LIMIT 50

　　{

　　ACCESS--> SELECT TOP 50 * FROM TAB_USERS ORDER BY RND([字段])

　　这条语句在ACCESS中的查询结果是正确的，但在ASP.NET等后台代码要写成如下：

　　Random random=new Random(System.Guid.NewGuid().GetHashCode());

　　int val=random.Next();

　　string sql="SELECT TOP 50 * FROM TAB_USERS ORDER BY RND("+(-r)+"字段)";

　　}