如何選擇MySQL中另一列的子集的列？

Question

我有這個問題的復雜版本（ 如何在mysql中選擇具有相同值集的列？ ）來處理。

在關系R（A，B，C）中，問題是要找出“具有4個或更多個常見B的A” 。 僅供參考：“ AB”是候選關鍵字。

我能做的就是這個

查詢：

select * from 
(select A, group_concat (B separator ', ') all_b's from R group by A having
(count(B))>3) p1  
join  
(select A, group_concat (B separator ', ') all_b's from R group by A having
(count(B))>3) p2  
on p1.all_b's = p2.all_b's and p1.A <> p2.A; 

輸出：

Null Set

但是，答案應該是別的。 知道如何處理嗎？

樣本數據：

A   B   C
a1  b1  asdas  
a1  b2  sdvsd    
a1  b3  sdfs  
a1  b4  evevr  
a2  b1  jdjd  
a2  b2  dkjlfnv  
a2  b3  sdfs  
a2  b4  evevr  
a2  b5  adfgaf  
a3  b1  sdfsdf  

預期產量

A   A  count
a1  a2  4

Answer 1

應該是這樣的：

SELECT
    first.A AS first_A,
    second.A AS second_A,
    COUNT(*) AS countSameBs
FROM
    R first
JOIN
    R second ON
    first.B = second.B AND
    first.A != second.A
GROUP BY
    first_A, second_A
HAVING
    countSameBs >= 4 AND
    first_A < second_A