如何快速變平SQL表

Question

我正在使用Presto。如果我有這樣一個表：

ID CATEGORY  VALUE 
1  a   ... 
1  b 
1  c 
2  a 
2  b 
3  b 
3  d 
3  e 
3  f 

你會如何轉換成以下，而無需編寫一個case語句爲每個組合？

ID  A  B C D E F 
1 
2 
3 

Answer 1

我從來沒有用過的Presto和文檔似乎相當薄，但基於this article它看起來像你可以做

SELECT 
    id, 
    kv['A'] AS A, 
    kv['B'] AS B, 
    kv['C'] AS C, 
    kv['D'] AS D, 
    kv['E'] AS E, 
    kv['F'] AS F 
FROM (
    SELECT id, map_agg(category, value) kv 
    FROM vtable 
    GROUP BY id 
) t 

雖然我建議在可能的話顯示層這樣做因爲你必須指定列。大多數報告工具和用戶界面網格都支持某種基於源數據創建列的動態透視。

Answer 2

我的2美分：

如果你知道 「可能」 的值：

SELECT 
    m['web'] AS web, 
    m['shopping'] AS shopping, 
    m['news'] AS news, 
    m['music'] AS music, 
    m['images'] AS images, 
    m['videos'] AS videos, 
    m[''] AS empty 
FROM (
SELECT histogram(data_tab) AS m 
FROM datahub 
WHERE 
    year = 2017 
    AND month = 5 
    AND day = 7 
    AND name = 'search' 
) searches 

沒有旋轉功能（還）！