Oracle SQL 分析函数 first、last、first_value、last_value、nth_value

--数据库版本SEAN@sean> select * from v$version;BANNER--------------------------------------------------------------------------------Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit ProductionPL/SQL Release 11.2.0.4.0 - ProductionCORE    11.2.0.4.0      ProductionTNS for Linux: Version 11.2.0.4.0 - ProductionNLSRTL Version 11.2.0.4.0 - Production--建表SEAN@sean> create table t    as    select 'A' || mod(level, 3) grp, level val, sysdate + level / 24 crt_date      from dual    connect by level <= 9;Table created.SEAN@sean> col grp for a10;SEAN@sean> alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';Session altered.SEAN@sean> select * from t order by grp,crt_date;GRP               VAL CRT_DATE---------- ---------- -------------------A0                  3 2017-07-25 11:13:41A0                  6 2017-07-25 14:13:41A0                  9 2017-07-25 17:13:41  --结果行1A1                  1 2017-07-25 09:13:41A1                  4 2017-07-25 12:13:41A1                  7 2017-07-25 15:13:41  --结果行2A2                  2 2017-07-25 10:13:41A2                  5 2017-07-25 13:13:41A2                  8 2017-07-25 16:13:41  --结果行39 rows selected.SEAN@sean> exec dbms_stats.gather_table_stats(user,'T');PL/SQL procedure successfully completed.--需求求每组时间(crt_date)最大的那个值(val),结果字段命令为 result_val 和 result_crt_date--如果 order by  t.crt_date 不能唯一确定排序，那么结果可能不正确。这时可以加入 rowid 辅助排序 order by  t.crt_date,rowid，本例假设 order by  t.crt_date 能唯一确定排序--方法1 row_number()SEAN@sean> select grp, val result_val, crt_date result_crt_date      from (select t.grp,                   t.val,                   t.crt_date,                   row_number() over(partition by t.grp order by crt_date desc) rn              from t)     where rn = 1;GRP        RESULT_VAL RESULT_CRT_DATE---------- ---------- -------------------A0                  9 2017-07-25 17:13:41A1                  7 2017-07-25 15:13:41A2                  8 2017-07-25 16:13:41--方法2 (last + order by asc)SEAN@sean> select t.grp,           max(t.val) keep(dense_rank last order by t.crt_date asc) result_val,           max(t.crt_date) keep(dense_rank last order by t.crt_date asc) result_crt_date      from t     group by t.grp;GRP        RESULT_VAL RESULT_CRT_DATE---------- ---------- -------------------A0                  9 2017-07-25 17:13:41A1                  7 2017-07-25 15:13:41A2                  8 2017-07-25 16:13:41--方法3 (first + order by desc)SEAN@sean> select t.grp,           max(t.val) keep(dense_rank first order by t.crt_date desc) result_val,           max(t.crt_date) keep(dense_rank first order by t.crt_date desc) result_crt_date      from t     group by t.grp;GRP        RESULT_VAL RESULT_CRT_DATE---------- ---------- -------------------A0                  9 2017-07-25 17:13:41A1                  7 2017-07-25 15:13:41A2                  8 2017-07-25 16:13:41--方法3.1 (first + order by desc) 的分析函数写法，每组返回多行数据SEAN@sean> select t.grp,t.val,t.crt_date,           max(t.val) keep(dense_rank first order by t.crt_date desc) over(partition by t.grp) result_val,           max(t.crt_date) keep(dense_rank first order by t.crt_date desc) over(partition by t.grp) result_crt_date      from t;GRP               VAL CRT_DATE            RESULT_VAL RESULT_CRT_DATE---------- ---------- ------------------- ---------- -------------------A0                  3 2017-07-25 11:13:41          9 2017-07-25 17:13:41A0                  9 2017-07-25 17:13:41          9 2017-07-25 17:13:41A0                  6 2017-07-25 14:13:41          9 2017-07-25 17:13:41A1                  1 2017-07-25 09:13:41          7 2017-07-25 15:13:41A1                  7 2017-07-25 15:13:41          7 2017-07-25 15:13:41A1                  4 2017-07-25 12:13:41          7 2017-07-25 15:13:41A2                  8 2017-07-25 16:13:41          8 2017-07-25 16:13:41A2                  2 2017-07-25 10:13:41          8 2017-07-25 16:13:41A2                  5 2017-07-25 13:13:41          8 2017-07-25 16:13:419 rows selected.--If you omit the windowing_clause entirely, then the default is RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.--对于分析函数，省略窗口子句，默认值是 RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW，也就是第一行到当前行--方法4 (last_value)select t.grp,t.val,t.crt_date,       last_value(t.val) over(partition by t.grp order by t.crt_date asc range between unbounded preceding and unbounded following) result_val,       last_value(t.crt_date) over(partition by t.grp order by t.crt_date asc range between unbounded preceding and unbounded following) result_crt_date  from t;--方法5 (first_value)select t.grp,t.val,t.crt_date,       first_value(t.val) ignore nulls over(partition by t.grp order by t.crt_date desc range between unbounded preceding and unbounded following) result_val,       first_value(t.crt_date) ignore nulls over(partition by t.grp order by t.crt_date desc range between unbounded preceding and unbounded following) result_crt_date  from t;--方法6 (nth_value)select t.grp,t.val,t.crt_date, nth_value(t.val, 1)  from last ignore nulls over(partition by t.grp order by t.crt_date asc rows between unbounded preceding and unbounded following) result_val,       nth_value(t.crt_date, 1)  from last ignore nulls over(partition by t.grp order by t.crt_date asc rows between unbounded preceding and unbounded following) result_crt_date  from t;总结：使用内置函数避免一些中间视图的构造和表的自连接