SQL - 如何转置？

发布于 2024-09-12 09:14:11 字数 1419 浏览 5 评论 0原文

我有类似于下表的内容：

================================================
| Id | UserId | FieldName     | FieldValue     |
=====+========+===============+================|
| 1  | 100    | Username      | John Doe       |
|----+--------+---------------+----------------|
| 2  | 100    | Password      | pass123!       |
|----+--------+---------------+----------------|
| 3  | 102    | Username      | Jane           |
|----+--------+---------------+----------------|
| 4  | 102    | Password      | $ecret         |
|----+--------+---------------+----------------|
| 5  | 102    | Email Address | [email protected] |
------------------------------------------------

我需要一个查询，该查询将为我提供如下结果：

==================================================
| UserId | Username  | Password | Email Address  |
=========+===========+===========================|
| 100    | John Doe  | pass123! |                |
|--------+-----------+----------+----------------|
| 102    | Jane      | $ecret   | [email protected] |
|--------+-----------+----------+----------------|

请注意，FieldName 中的值不限于用户名、密码和电子邮件地址。它们可以是任何东西，因为它们是用户定义的。

有没有办法在 SQL 中做到这一点？

原文

I have something similar to the following table:

================================================
| Id | UserId | FieldName     | FieldValue     |
=====+========+===============+================|
| 1  | 100    | Username      | John Doe       |
|----+--------+---------------+----------------|
| 2  | 100    | Password      | pass123!       |
|----+--------+---------------+----------------|
| 3  | 102    | Username      | Jane           |
|----+--------+---------------+----------------|
| 4  | 102    | Password      | $ecret         |
|----+--------+---------------+----------------|
| 5  | 102    | Email Address | [email protected] |
------------------------------------------------

I need a query that will give me a result like this:

==================================================
| UserId | Username  | Password | Email Address  |
=========+===========+===========================|
| 100    | John Doe  | pass123! |                |
|--------+-----------+----------+----------------|
| 102    | Jane      | $ecret   | [email protected] |
|--------+-----------+----------+----------------|

Note that the values in FieldName are not limited to Username, Password, and Email Address. They can be anything as they are user defined.

Is there a way to do this in SQL?

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梦忆晨望 2024-09-19 09:14:11

MySQL 不支持 ANSI PIVOT/UNPIVOT 语法，因此您只能使用：

  SELECT t.userid
         MAX(CASE WHEN t.fieldname = 'Username' THEN t.fieldvalue ELSE NULL END) AS Username,
         MAX(CASE WHEN t.fieldname = 'Password' THEN t.fieldvalue ELSE NULL END) AS Password,
         MAX(CASE WHEN t.fieldname = 'Email Address' THEN t.fieldvalue ELSE NULL END) AS Email
    FROM TABLE t
GROUP BY t.userid

如您所见，需要为每个值定义 CASE 语句。要使其动态化，您需要使用 MySQL 的准备语句（动态 SQL）语法。

MySQL doesn't support ANSI PIVOT/UNPIVOT syntax, so that leave you to use:

  SELECT t.userid
         MAX(CASE WHEN t.fieldname = 'Username' THEN t.fieldvalue ELSE NULL END) AS Username,
         MAX(CASE WHEN t.fieldname = 'Password' THEN t.fieldvalue ELSE NULL END) AS Password,
         MAX(CASE WHEN t.fieldname = 'Email Address' THEN t.fieldvalue ELSE NULL END) AS Email
    FROM TABLE t
GROUP BY t.userid

As you can see, the CASE statements need to be defined per value. To make this dynamic, you'd need to use MySQL's Prepared Statement (dynamic SQL) syntax.

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渔村楼浪 2024-09-19 09:14:11

您可以使用 GROUP_CONCAT

（未经测试）

SELECT UserId, 
GROUP_CONCAT( if( fieldname = 'Username', fieldvalue, NULL ) ) AS 'Username', 
GROUP_CONCAT( if( fieldname = 'Password', fieldvalue, NULL ) ) AS 'Password', 
GROUP_CONCAT( if( fieldname = 'Email Address', fieldvalue, NULL ) ) AS 'Email Address', 
FROM table  
GROUP BY UserId

You could use GROUP_CONCAT

(untested)

SELECT UserId, 
GROUP_CONCAT( if( fieldname = 'Username', fieldvalue, NULL ) ) AS 'Username', 
GROUP_CONCAT( if( fieldname = 'Password', fieldvalue, NULL ) ) AS 'Password', 
GROUP_CONCAT( if( fieldname = 'Email Address', fieldvalue, NULL ) ) AS 'Email Address', 
FROM table  
GROUP BY UserId

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