ORACLE中使用row_number over()排序

from:http://blog.csdn.net/iw1210/article/details/11937085

意图：实现select top 1 * from tablename

Oracle 中的 ROW_NUMBER() OVER() 分析函数的用法

ROW_NUMBER() OVER(partition by col1 order by col2) 表示根据col1分组，在分组内部根据col2排序，而此函数计算的值就表示每组内部排序后的顺序编号（组内是连续且唯一的）。

举例：

SQL> DESC T1;

Name                                           Null?    Type
----------------------------------------- -------- ----------------------------
ID                                                    NUMBER
NAME                                                    VARCHAR2(10)
DATE1                                                    DATE

SQL> SELECT * FROM T1;
        ID NAME                           DATE1
---------- ------------------------------ ------------------
       101 aaa                                  09-SEP-13

101 bbb                                  10-SEP-13

101 ccc                                  11-SEP-13

102 ddd                                  08-SEP-13

102 eee                                  11-SEP-13

SQL> SELECT ID,NAME,DATE1,ROW_NUMBER() OVER(partition by ID order by DATE1 desc) as RN FROM T1;

ID NAME               DATE1               RN
---------- ------------------------------ ------------------ ----------
       101 ccc                  11-SEP-13              1
       101 bbb                  10-SEP-13              2
       101 aaa                  09-SEP-13              3
       102 eee                  11-SEP-13              1
       102 ddd                  08-SEP-13              2

把上面语句作为一个子表语句，嵌入到另一条语句中：

SQL> SELECT ID,NAME,DATE1 FROM (SELECT ID,NAME,DATE1,ROW_NUMBER() OVER(partition by ID order by DATE1 desc) as RN FROM T1)T WHERE T.RN=1;

ID NAME                           DATE1
---------- ------------------------------ ------------------
       101 ccc                                  11-SEP-13
       102 eee                                  11-SEP-13