MySQL：在GROUP BY之后添加多个列？

Question

我正在尝试执行以下操作：

SELECT 
  customer_entity.email,
  customer_entity.website_id 
FROM
  customer_entity 
  INNER JOIN 
    (SELECT 
      email 
    FROM
      customer_entity 
    GROUP BY email 
    HAVING COUNT(email) > 1) dupes 
    ON customer_entity.email = dupes.email  
GROUP BY customer_entity.`entity_id` 
ORDER BY customer_entity.email ;

以上查询返回以下结果：

email               website_id
[email protected]             1
[email protected]             2
[email protected]             3
[email protected]             4
[email protected]            1
[email protected]            2
[email protected]            4
[email protected]              1
[email protected]              2
[email protected]              3

但是我想要以下格式的数据：

email           website1    website2    website3    website4

[email protected]         1           2           3           4 
[email protected]        1           2           null        4
[email protected]          1           2           3           null

在这种情况下可以吗？

谢谢

Answer 1

select
    email,
    max(website_id = 1) website_1,
    max(website_id = 2) website_2,
    max(website_id = 3) website_3,
    max(website_id = 4) website_4
from customer_entity
group by email
having count(*) > 1
order by email

请注意，这简化了您的原始查询-此处不需要自我联接。

此外，这在每列中输入0/1值，以指示该网站是否存在给定的电子邮件-我发现它比重复该列中的网站ID更有意义。