顯示類值

Question

我不知道如何會從這個表中的SQLite寫查詢：

image_id | feature_id | feature_val 
_________|____________|____________ 
    12  | 1  |  41 
    12  | 2  |  82 
    12  | 3  |  75 

    74  | 1  |  23 
    74  | 2  |  35 
    74  | 3  |  72 

返回的結果表：

image_id | feature_1_val | feature_2_val | feautre_3_val 
_________|_________________|_______________|_______________ 
    12  |  41  |  82  |  75 
    74  |  23  |  35  |  72  

我們可以假設圖像特徵的數量是恆定的，等於3. 我會非常感謝您的幫助。

Answer 1

將行轉換爲列的過程稱爲pivoting。客戶端最好完成。例如，Microsoft Excel通過數據透視表提供了廣泛的支持。

但是在SQL中這不是不可能的。下面是一個使用max(case(招一個例子：

select image_id 
,  max(case when feature_id = 1 then feature_val end) as feature_1_val 
,  max(case when feature_id = 2 then feature_val end) as feature_2_val 
from YourTable 
group by 
     image_id 

Answer 2

鑑於該問題列出的約束，你可以創建一個不雅的，但可行的子查詢使用方法：

SELECT it.image_id, 
     (SELECT feature_val FROM ImageTable WHERE ImageTable.image_id = it.image_id AND ImageTable.feature_id = 1) AS feature_1_val, 
     (SELECT feature_val FROM ImageTable WHERE ImageTable.image_id = it.image_id AND ImageTable.feature_id = 2) AS feature_2_val, 
     (SELECT feature_val FROM ImageTable WHERE ImageTable.image_id = it.image_id AND ImageTable.feature_id = 3) AS feature_3_val 
    FROM ImageTable AS it 
    GROUP BY it.image_id