集團具有clearification

Question

這是我嘗試acheieve（寫在imaginery SQL）：

SELECT A, B, C, AVG( `Value`) AS Value 
FROM T 
GROUP BY A, B, C 
HAVING AT LEAST TWO DIFFRENT C VALUES PER DISTINCT A,B 
ORDER BY A, B, C 

是否有可能改寫正確的MySQL這樣的HAVING條件？

起初我以爲這會工作：

SELECT A, B, C, AVG( `Value`) AS Value 
    FROM T 
    GROUP BY A, B, C 
    HAVING COUNT(C)>2 
    ORDER BY A, B, C 

但由於爲同A，B，C組合其不可能2不同勢C值它不，那​​麼你會怎麼做呢？

Answer 1

您可以將您的查詢分爲兩個不同的查詢：

1. 有每個不同的至少兩種不同的C值，B

   select A, B 
   from T 
   group by A, B 
   having count(DISTINCT C)>1 
   

2. 您的最終查詢

   select A, B, C, avg(Value) 
   from T 
   where 
       (A, B) in (
       select A, B 
       from T 
       group by A, B 
       having count(DISTINCT C)>1 
   ) 
   group by 
       A, B, C 
   

Answer 2

你可以嘗試使用WHERE聲明，而不是HAVING

SELECT A, B, C, AVG( `Value`) AS Value 
FROM T 
WHERE 
    (
    SELECT COUNT(DISTINCT C) 
    FROM T1 
    WHERE T1.A = T.A AND T1.B = T.B 
    ) >= 2 
GROUP BY A, B, C 
ORDER BY A, B, C