在SQL Server里面有top关键字可以很方便的取出前N条记录，但是Oracle里面却没有top的使用，类似实现取出前N条记录的简单方法如下：
方法1：利用ROW_NUMBER函数
取出前5条记录：
SELECT NO FROM (
SELECT ROW_NUMBER() OVER (ORDER BY NO) RNO, NO FROM ROWNUM_TEST
)WHERE RNO <= 5 ORDER BY NO ;
取出中间5条记录：
SELECT NO FROM (
SELECT ROW_NUMBER() OVER (ORDER BY NO) RNO, NO FROM ROWNUM_TEST
) WHERE RNO BETWEEN 5 AND 10 ORDER BY NO;
方法2：利用子查询
取出前5条记录：
SELECT NO FROM (
SELECT NO FROM ROWNUM_TEST ORDER BY NO
)WHERE ROWNUM <= 5 ORDER BY NO ;
取出中间5条记录：
SELECT NO FROM (
SELECT NO, ROWNUM RNO FROM (
SELECT NO FROM ROWNUM_TEST ORDER BY NO
)
) WHERE RNO BETWEEN 5 AND 10 ORDER BY NO;