有重复组时选择最后一组的第一条记录

Posted 2023-05-09

【中文标题】有重复组时选择最后一组的第一条记录

【英文标题】：Select the first record of the last group when there are repeating groups

【发布时间】：2018-12-25 20:01:52

【问题描述】：

尝试为每个 POLICY_ID 选择最新重复 STATUS 组的第一条记录。我该怎么做？

编辑/注意：可以有两个以上的状态重复，如最后三行所示。

查看数据：

期望的输出：

用于数据的 SQL：

--drop table mytable;
create table mytable (ROW_ID Number(5), POLICY_ID Number(5), 
                      CHANGE_NO Number(5), STATUS VARCHAR(50), CHANGE_DATE DATE);

insert into mytable values (  81, 1, 1, 'A', date '2018-01-01');
insert into mytable values (  95, 1, 2, 'A', date '2018-01-02');
insert into mytable values ( 100, 1, 3, 'B', date '2018-01-03');
insert into mytable values ( 150, 1, 4, 'C', date '2018-01-04');
insert into mytable values ( 165, 1, 5, 'A', date '2018-01-05');
insert into mytable values ( 175, 1, 6, 'A', date '2018-01-06');
insert into mytable values ( 599, 2, 1, 'S', date '2018-01-11');
insert into mytable values ( 602, 2, 2, 'S', date '2018-01-12');
insert into mytable values ( 611, 2, 3, 'S', date '2018-01-13');
insert into mytable values ( 629, 2, 4, 'T', date '2018-01-14');
insert into mytable values ( 720, 2, 5, 'U', date '2018-01-15');
insert into mytable values ( 790, 2, 6, 'S', date '2018-01-16');
insert into mytable values ( 812, 2, 7, 'S', date '2018-01-17');
insert into mytable values ( 825, 2, 8, 'S', date '2018-01-18');

select * from mytable;

【问题讨论】：

您使用哪个版本的 Oracle？如果是 12c，那么您可以使用 MATCH_RECOGNIZE

【参考方案1】：

嗯。 . .

select t.*
from (select t.*,
             row_number() over (partition by policy_id order by change_date asc) as seqnum
      from t
      where not exists (select 1
                        from t t2
                        where t2.policy_id = t.policy_id and
                              t2.status <> t.status and
                              t2.change_date > t.change_date
                       )
     ) t
where seqnum = 1;

内部子查询查找所有行，其中 - 对于给定的策略编号 - 没有具有不同状态的后续行。这定义了最后一组记录。

然后它使用row_number() 枚举行。这些外部查询为每个policy_number 选择第一行。

【讨论】：

成功了，谢谢！你能再解释一下“选择1”部分吗？但是，您没有从 t2 中选择任何列，在 where 子句中您使用了该表中的列。

@kzmlbyrk 。 . . not exists 检查子查询是否返回了 row。行中的值没有区别。 select 1 是最容易输入的。

【参考方案2】：

您可以使用 LEAD 和 LAG 函数来识别开始“重复”的行。条件 status <> previous status and status = next status 将识别此类行。

SELECT *
FROM (
    SELECT cte1.*, ROW_NUMBER() OVER (PARTITION BY POLICY_ID ORDER BY CHANGE_DATE DESC) AS rn
    FROM (
            SELECT mytable.*, CASE WHEN
                STATUS <> LAG(STATUS, 1, '!') OVER (PARTITION BY POLICY_ID ORDER BY CHANGE_DATE) AND
                STATUS = LEAD(STATUS) OVER (PARTITION BY POLICY_ID ORDER BY CHANGE_DATE)
            THEN 1 END AS toselect
            FROM mytable
    ) cte1
    WHERE toselect = 1
) cte2
WHERE rn = 1

【参考方案3】：

如果你使用Oracle 12c，你可以使用MATCH_RECOGNIZE：

SELECT ROW_ID, POLICY_ID, CHANGE_NO, STATUS, CHANGE_DATE
FROM mytable
MATCH_RECOGNIZE (
  PARTITION BY POLICY_ID
  ORDER BY CHANGE_DATE
  MEASURES MATCH_NUMBER() m,FIRST(R.ROW_ID) r
  ALL ROWS PER MATCH
  PATTERN (R+)
  DEFINE R AS STATUS=NEXT(STATUS)
) MR
WHERE ROW_ID = R
ORDER BY ROW_NUMBER() OVER(PARTITION BY POLICY_ID ORDER BY M DESC)
FETCH FIRST 1 ROW WITH TIES;

db<>fiddle demo

---

或者：

SELECT *
FROM mytable 
MATCH_RECOGNIZE (
  PARTITION BY POLICY_ID
  ORDER BY CHANGE_DATE DESC
  MEASURES MATCH_NUMBER() m
           ,LAST(R.ROW_ID) ROW_ID
           ,LAST(R.STATUS) STATUS
           ,LAST(R.CHANGE_NO) CHANGE_NO
           ,LAST(R.CHANGE_DATE) CHANGE_DATE
  ONE ROW PER MATCH
  PATTERN (R+)
  DEFINE R AS STATUS=PREV(STATUS)
) MR
WHERE M = 1

db<>fiddle demo2

【参考方案4】：

match_recognize 的另一种方法：

select row_id, policy_id, change_no, status, change_date
from   mytable
       match_recognize (
           partition by policy_id
           order by change_date
           measures
             strt.row_id as row_id
           , strt.change_no as change_no
           , strt.change_date as change_date
           , strt.status as status
           pattern (strt unchanged* final)
           define
             unchanged as next(unchanged.status) = prev(unchanged.status)
           , final as next(final.status) is null
      ) mr
order by mr.policy_id;