Oracle SQL -- 对一个组的分析函数？

Question

我的桌子：

ID   NUM   VAL
1    1     Hello
1    2     Goodbye
2    2     Hey
2    4     What's up?
3    5     See you

如果我想返回每个 ID 的最大数量，那真的很干净：

SELECT MAX(NUM) FROM table GROUP BY (ID)

但是，如果我想获取与每个 ID 的每个数字的最大值相关的值怎么办？

为什么我做不到：

SELECT MAX(NUM) OVER (ORDER BY NUM) FROM table GROUP BY (ID) 

为什么这是一个错误？我希望这个选择按 ID 分组，而不是为每个窗口单独分区...

编辑：错误是“不是 GROUP BY 表达式”。

Answer 1

您或许可以使用MAX() KEEP(DENSE_RANK LAST...) 函数：

with sample_data as (
  select 1 id, 1 num, 'Hello' val from dual union all
  select 1 id, 2 num, 'Goodbye' val from dual union all
  select 2 id, 2 num, 'Hey' val from dual union all
  select 2 id, 4 num, 'What''s up?' val from dual union all
  select 3 id, 5 num, 'See you' val from dual)
select id, max(num), max(val) keep (dense_rank last order by num)
from sample_data
group by id;

Answer 2

当你使用开窗功能时，你不需要再使用 GROUP BY，这样就足够了：

select id, 
     max(num) over(partition by id) 
from x 

其实不用加窗函数也可以得到结果：

select * 
from x
where (id,num) in
  (
     select id, max(num) 
     from x 
     group by id
  )

输出：

ID  NUM VAL
1   2   Goodbye
2   4   What's up
3   5   SEE YOU

http://www.sqlfiddle.com/#!4/a9a07/7

---

如果你想使用窗口功能，你可以这样做：

select id, val, 
     case when num =  max(num) over(partition by id) then
        1
     else
        0
     end as to_select
from x 
where to_select = 1

或者这个：

select id, val 
from x 
where num =  max(num) over(partition by id) 

但既然不允许这样做，你必须这样做：

with list as
(
  select id, val, 
     case when num =  max(num) over(partition by id) then
        1
     else
        0
     end as to_select
  from x
)
select * 
from list 
where to_select = 1

http://www.sqlfiddle.com/#!4/a9a07/19

Answer 3

如果您要获取包含来自MAX(num) GROUP BY id 的值的行，这往往是一种常见模式...

WITH
  sequenced_data
AS
(
  SELECT
    ROW_NUMBER() OVER (PARTITION BY id ORDER BY num DESC) AS sequence_id,
    *
  FROM
    yourTable
)
SELECT
  *
FROM
  sequenced_data
WHERE
  sequence_id = 1

编辑

我不知道 TeraData 是否会允许这样做，但逻辑似乎是有道理的......

SELECT
  *
FROM
  yourTable
WHERE
  num = MAX(num) OVER (PARTITION BY id)

或许……

SELECT
  *
FROM
(
  SELECT
    *,
    MAX(num) OVER (PARTITION BY id) AS max_num_by_id
  FROM
    yourTable
)
  AS sub_query
WHERE
  num = max_num_by_id 

这与我之前的回答略有不同；如果多个记录与同一个MAX(num) 绑定，这将返回所有记录，另一个答案将只返回一个。

编辑

在您提出的 SQL 中，错误与 OVER() 子句包含不在您的 GROUP BY 中的字段有关。这就像尝试这样做......

SELECT id, num FROM yourTable GROUP BY id

num 无效，因为返回的每一行在该字段中可能有多个值（返回的行由GROUP BY id 定义）。

同样，您不能将num 放在OVER() 子句中。

SELECT

  id,

  MAX(num),                <-- Valid as it is an aggregate

  MAX(num)                 <-- still valid
  OVER(PARTITION BY id),   <-- Also valid, as id is in the GROUP BY

  MAX(num)                 <-- still valid
  OVER(PARTITION BY num)   <-- Not valid, as num is not in the GROUP BY

FROM
  yourTable
GROUP BY
  id

当你不能在 OVER() 子句中指定某些内容时，请参阅此问题，以及显示何时（我认为）你可以的答案：over-partition-by-question