SQL - 如何转置？ [英] SQL - How to transpose?
本文介绍了SQL - 如何转置？的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我有类似下表：
================================================
| 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 | jane@email.com |
------------------------------------------------
我需要一个查询，它会给我一个这样的结果：
I need a query that will give me a result like this:
==================================================
| UserId | Username | Password | Email Address |
=========+===========+===========================|
| 100 | John Doe | pass123! | |
|--------+-----------+----------+----------------|
| 102 | Jane | $ecret | jane@email.com |
|--------+-----------+----------+----------------|
请注意，FieldName中的值不限于用户名，密码和电子邮件地址。
Note that the values in FieldName are not limited to Username, Password, and Email Address. They can be anything as they are user defined.
在SQL中有没有办法做到这一点？
Is there a way to do this in SQL?
推荐答案
MySQL不支持ANSI PIVOT / UNPIVOT语法，因此您可以使用：
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
正如你所看到的，CASE语句需要根据值来定义。要启用此功能，您需要使用 MySQL的Prepared Statement（动态SQL）语法。
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.
这篇关于SQL - 如何转置？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
查看全文
