SQL查詢顯示數據表中最近一行的數據，或者爲空

Question

我有一個Oracle數據庫表結構，其行數通過採訪過程填充，每個「客戶」都可以多次訪問採訪過程，即使是同一天。該表結構如下：

Table: Customers 
------------- 
CustomerId 
... 

Table: Questions 
------------- 
QuestionId | QuestionText 
0   | Last Location? 
1   | Last Color? 
2   | Last Food? 
3   | Last Drink? 

Table: Answers 
------------- 
Id | CustomerId | QuestionId | AnswerText | Created_On 
1 0   0   Chicago  08/15/2017 7:56:34 AM 
2 0   0   Laramie  08/16/2017 9:27:23 AM 
3 0   0   Null   08/17/2017 6:34:56 AM 
4 0   1   Null   08/15/2017 7:56:34 AM 
5 0   1   Green  08/16/2017 9:27:23 AM 
6 0   1   Blue   08/17/2017 6:34:56 AM 
7 0   2   Pizza  08/15/2017 7:56:34 AM 
8 0   2   Null   08/16/2017 9:27:23 AM 
9 0   2   Null   08/17/2017 6:34:56 AM 
10 0   3   Null   08/15/2017 7:56:34 AM 
11 0   3   Null   08/16/2017 9:27:23 AM 
12 0   3   Null   08/17/2017 6:34:56 AM 

目前我們顯示「最後一次採訪」通過查詢數據爲最近的時間戳對每一個問題，並顯示結果，它是否有一個值或爲空，這樣：

LastAnswer_QueryResult 
------------- 
Id | CustomerId | QuestionId | AnswerText | Created_On 
3 0   0   Null   08/17/2017 6:34:56 AM 
6 0   1   Blue   08/17/2017 6:34:56 AM 
9 0   2   Null   08/17/2017 6:34:56 AM 
12 0   3   Null   08/17/2017 6:34:56 AM 

的新要求，是展示最新的答案：對於使用值的每個問題，或者爲空，如果這個問題從來沒有得到回答。如果我能得到查詢後的結果爲樣本數據上面看起來像：

MostRecentAnswer_QueryResult 
Id | CustomerId | QuestionId | AnswerText | Created_On 
2 0   0   Laramie  08/16/2017 9:27:23 AM 
6 0   1   Blue   08/17/2017 6:34:56 AM 
7 0   2   Pizza  08/15/2017 7:56:34 AM 
12 0   3   Null   08/17/2017 6:34:56 AM 

到目前爲止，我能想出的最好的辦法是最古老的行插入到一個臨時表，然後在環如果它們存在較新的時間戳，則更新這些值。然後完成後，使用最新的時間戳更新任何空值。有沒有一種方法，我可以做到這一點沒有循環和插入臨時表？

Answer 1

救援分析功能！分區爲customerid和questionid，並將row_number()分配給每個分區中的行。如果我們不必擔心answertext is null，我們只需訂購created_on desc。

要處理null，我們首先要判斷answertext是否爲null，而不是null。這可以通過case表達式輕鬆完成（參見下文）。

然後在外部查詢中，我們選擇row_number爲1的行（在customerid和questionid的每個組合內）。

select id, customerid, questionid, answertext, ts 
from (
     select id, customerid, questionid, answertext, ts, 
       row_number() over (partition by customerid, questionid 
             order by case when answertext is not null 
                 then 0 end, 
               created_on desc 
           ) as rn 
     from answers 
     ) 
where rn = 1 
; 

Answer 2

你可以試試：

select MAX(a.id) ID, a.CustomerId , q.QuestionId,LISTAGG(AnswerText,' * ') WITHIN GROUP (ORDER BY ID) AnswerText, max(a.Created_On) Created_On 
    from Questions q 
    left join Answers a 
    on a.QuestionId = q.QuestionId 
    where a.AnswerText is not null 
    group by a.CustomerId , q.QuestionId 
    UNION ALL 
    select MAX(a.id) ID, a.CustomerId , q.QuestionId ,LISTAGG(AnswerText,' *') WITHIN GROUP (ORDER BY ID) AnswerText, max(a.Created_On) Created_On 
    from Questions q 
    left join Answers a 
    on a.QuestionId = q.QuestionId 
    where a.AnswerText is null AND 
    Q.QuestionId NOT IN(
     select q.QuestionId 
     from Questions q 
     left join Answers a 
     on a.QuestionId = q.QuestionId 
     where a.AnswerText is not null 
     group by a.CustomerId , q.QuestionId 
    ) 
    group by a.CustomerId , q.QuestionId 
    ORDER BY QuestionId 

之後，你會得到之間的「*」