MYSQL 根据特定列选择多个列答案

【问题标题】：MYSQL Select multiple columns depending on a specific columnMYSQL 根据特定列选择多个列
【发布时间】：2017-08-31 00:36:40
【问题描述】：

我想做一个 SELECT 请求，根据 PLATFORM 值设法获得 2 列 VALUE（桌面和移动）。

这是一个示例表：

+----+---------+------+----------+-------+
| ID | PROJECT | NAME | PLATFORM | VALUE |
+----+---------+------+----------+-------+
|  1 |       1 | Foo  | desktop  |     1 |
|  2 |       1 | Foo  | mobile   |    42 |
|  3 |       1 | Bar  | desktop  |     3 |
|  4 |       1 | Bar  | mobile   |    10 |
|  5 |       2 | Foo  | desktop  |     2 |
|  6 |       2 | Bar  | mobile   |     9 |
+----+---------+------+----------+-------+

期望的输出：

+---------+------+---------+--------+
| PROJECT | NAME | DESKTOP | MOBILE |
+---------+------+---------+--------+
|       1 | Foo  | 1       | 42     |
|       1 | Bar  | 3       | 10     |
|       2 | Foo  | 2       | NULL   |
|       2 | Bar  | NULL    | 9      |
+---------+------+---------+--------+

我尝试了什么：

SELECT project, name,
(CASE platform WHEN 'desktop' THEN value END) AS "desktop",
(CASE platform WHEN 'mobile' THEN value END) AS "mobile"
FROM test
GROUP BY name, project
ORDER BY project, value ASC

+---------+------+---------+--------+
| project | name | desktop | mobile |
+---------+------+---------+--------+
|       1 | Foo  | 1       | NULL   |
|       1 | Bar  | 3       | NULL   |
|       2 | Foo  | 2       | NULL   |
|       2 | Bar  | NULL    | 9      |
+---------+------+---------+--------+

【问题讨论】：

你非常接近 :-)
你有什么建议吗？ D：@Sebas

标签： mysql select merge group-by nested-select

【解决方案1】：

试试这个：

SELECT project, NAME, MAX(desktop) AS desktop, MAX(mobile) AS mobile FROM (
    SELECT project, NAME,
    (CASE platform WHEN 'desktop' THEN VALUE END) AS "desktop",
    (CASE platform WHEN 'mobile' THEN VALUE END) AS "mobile"
    FROM test
    ) AS aa
GROUP BY aa.NAME, aa.project
ORDER BY aa.project

说明：

首先你对所有数据进行选择（aa），根据平台内容扩展价值。

然后您使用该选择作为分组数据的来源。

结果：

project name   desktop mobile
1       Foo       1       42
1       Bar       3       10
2       Foo       2       NULL
2       Bar       NULL    9

【讨论】：