如何在 mysql 中拆分/取消透视 group_concat？

Question

我目前有一个视图，其中有一个名为 alt_email_contact 的列，我使用 group_concat 函数来获取与一个联系人关联的多封电子邮件。但是，我希望能够提取每封电子邮件并为每封电子邮件创建一个单独的列。

例子：

id    email
 1     SkyW@gmail.com, SW@gmail.com, WW@gmail.com, WalterW@gmail.com 

电子邮件的数量可能会从一个用户更改为另一个用户，因此每个用户不会总是有四封电子邮件。我想为每封电子邮件创建一个新列，如下所示：

id      email_1         email_2        email_3        email_4 
1       SkyW@gmail.com  SW@gmail.com   WW@gmail.com   WalterW@gmail.com 

（我正在使用 phpmyadmin）我希望能够修改我的视图以包含每个用户的可变数量的电子邮件。

Answer 1

您可以使用substring_index() 来实现您想要的。它不是很漂亮，但它会起作用：

select id,
       substring_index(emails, ', ', 1) as email_1
       (case when length(emails) - length(replace(emails, ',', '')) >= 1
             then substring_index(substring_index(emails, ', ', 2), ', ', -1)
        end) as email_2,
       (case when length(emails) - length(replace(emails, ',', '')) >= 2
             then substring_index(substring_index(emails, ', ', 3), ', ', -1)
        end) as email_3,
       (case when length(emails) - length(replace(emails, ',', '')) >= 3
             then substring_index(substring_index(emails, ', ', 4), ', ', -1)
        end) as email_4,
       (case when length(emails) - length(replace(emails, ',', '')) >= 4
             then substring_index(substring_index(emails, ', ', 5), ', ', -1)
        end) as email_5     
from table t;

如果您愿意，可以将这些值插入到另一个表中。

Answer 2

不要使用那个视图，因为GROUP_CONCAT() 已经破坏了规范化。您想使用 MySQL 有限的 SQL 功能来模拟数据透视表。

假设您的视图基于如下所示的Contacts 表：

CREATE TABLE Contacts
( id INTEGER NOT NULL
, alt_email_contact VARCHAR(256) NOT NULL
);

改为创建这个帮助视图（基本上是 RANK() OVER (PARTITION BY id ORDER BY alt_email_contact)，除了 MySQL 不支持 RANK()）：

CREATE VIEW NumberedContacts AS
    SELECT c1.id, c1.alt_email_contact, COUNT(*) AS rank
        FROM Contacts c1
            INNER JOIN Contacts c2
                ON c2.id = c1.id AND
                   c1.alt_email_contact >= c2.alt_email_contact
        GROUP BY c1.id, c1.alt_email_contact;

然后您可以编写此查询或视图，它会为您提供最多 5 个备用电子邮件地址，按字母顺序排列：

CREATE VIEW ContactsForImport AS
    SELECT c1.id
         , c1.alt_email_contact AS email_1
         , c2.alt_email_contact AS email_2
         , c3.alt_email_contact AS email_3
         , c4.alt_email_contact AS email_4
         , c5.alt_email_contact AS email_5
        FROM NumberedContacts AS c1
            LEFT OUTER JOIN NumberedContacts AS c2
                ON c1.id = c2.id AND c2.rank = 2
            LEFT OUTER JOIN NumberedContacts AS c3
                ON c1.id = c3.id AND c3.rank = 3
            LEFT OUTER JOIN NumberedContacts AS c4
                ON c1.id = c4.id AND c4.rank = 4
            LEFT OUTER JOIN NumberedContacts AS c5
                ON c1.id = c5.id AND c5.rank = 5
        WHERE c1.rank = 1;

SQL Fiddle