如何在mysql中拆分/取消透視group_concat？

Question

我目前在一個視圖中有一列名為alt_email_contact的視圖，並且使用group_concat函數來獲取與一個聯系人關聯的多封電子郵件。 但是，我希望能夠提取每個電子郵件並為每個電子郵件創建一個單獨的列。

例：

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

電子郵件數量可能會從一個用戶更改為另一個用戶，因此每個用戶不會總是有四個電子郵件。 我想為每封電子郵件創建一個新列，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）我希望能夠修改我的視圖以包含每位用戶可變數量的電子郵件。

Answer 1

您可以使用substring_index()實現所需的功能。 它不是很漂亮，但是可以工作：

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;

如果願意，可以將這些值插入另一個表中。

Answer 2

不要使用該視圖，因為GROUP_CONCAT()已經破壞了規范化。 您想使用MySQL的有限SQL功能來模擬數據透視表。

假設您的視圖基於如下所示的“ Contacts表：

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

而是創建此幫助程序視圖（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，您可以編寫此查詢或視圖，該查詢或視圖最多提供5個備用電子郵件地址，按字母順序排列：

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小提琴