如何在mysql中选择包含另一列子集的列？

Question

我有一个复杂版本的问题（How to select columns with same set of values in mysql?）来处理。

在关系R（A，B，C）中，问题是找出“有4个或更多共同B的A”。仅供参考：“AB”是候选键。

我能做的就是这个

查询：

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; 

输出：

Null Set

但是，答案应该是别的。 知道怎么处理这个吗？

示例数据：

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  

预期输出

A   A  count
a1  a2  4

Answer 1

应该是这样的：

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