對於列A，B，C的SQL：按A組計算，B的計算份額等於每個A的C

Question

數據庫是Oracle。我有三列，A，B和C的單個表

• A是字符串列
• B是int列
• C是int列

我需要一個SQL選擇上的A基團，並返回份額（以百分數表示），其中B = C = [特定int值]爲A的每個值

喜歡的東西

SELECT A, percentBEqC 
... 
GROUP ON A 

Answer 1

SELECT A 
    , COUNT(CASE WHEN B = C AND C = 256 
        THEN 1 
       END 
      ) 
    /COUNT(*) AS percentBEqC 
FROM YourTable 
GROUP BY A 

您可能想要檢查B = 256 AND C = 256是否有更好的性能。

---

你的問題的另一種方法是：

SELECT ta.A 
    , COALESCE(cnt,0)/cntAll AS percentBEqC 
FROM 
    (SELECT A 
      , COUNT(*) AS cntAll 
     FROM YourTable 
     GROUP BY A 
    ) ta 
    LEFT JOIN 
    (SELECT A 
      , COUNT(*) AS cnt 
     FROM YourTable 
     WHERE B = 256 AND C = 256 
     GROUP BY A 
    ) ts 
    ON ts.A = ta.A 

Answer 2

另一種方式：
_{..once語法是正確的;/}

SELECT A, (COUNT(*)/(SELECT COUNT(*) FROM mytbl b WHERE b.A = a.A)) AS percentBEqC 
    FROM mytbl a 
WHERE B = particular_value 
    AND C = particular_value 
GROUP BY A; 

只返回行，其中percentBEqC > 0，