根据特定列选择多个列的MYSQL查询

Question

我希望进行一个SELECT请求，该请求可以根据PLATFORM的值获取2列VALUE（DESKTOP和MOBILE）。

下面是一个示例表格：

+----+---------+------+----------+-------+
| 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      |
+---------+------+---------+--------+

我尝试过的方法：

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      |
+---------+------+---------+--------+

- Jack

---

你非常接近了 :-) - Sebas

你有什么技巧吗？D：@Sebas - Jack

Answer 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

解释：

首先，您需要根据平台的内容扩展值，从而选择所有数据（aa）。

然后，您将使用该选择作为分组数据的起点。

结果：

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

- roetnig