mysql - 如何在mysql中选择具有另一列子集的列？

Question

我要处理这个问题的复杂版本 (How to select columns with same set of values in mysql?)。

在关系 R(A,B,C) 中，问题是找出“A 与 4 个或更多常见 B”。仅供引用:“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，答案应该是别的东西。 知道如何处理这个吗？

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