如何编写SQL实现单元格及下方Installs值的拼接展示？

现有表结构如下：

| Days Since Install | Installs |
| ------------------ | -------- |
|         1          | A        |
|         2          | B        |
|         3          | C        |
|         4          | D        |

需要生成目标表，其中每行的Installs字段为当前行及下方所有行的Installs值以+拼接，结果如下：

| Days Since Install | Installs |
| ------------------ | -------- |
|         1          | A+B+C+D  |
|         2          | B+C+D    |
|         3          | C+D      |
|         4          | D        |

可以编写SQL语句实现该需求，不同数据库的实现方式如下：

MySQL（8.0及以上版本） #

利用窗口函数结合GROUP_CONCAT实现：

SELECT 
    `Days Since Install`,
    GROUP_CONCAT(`Installs` SEPARATOR '+') OVER (
        ORDER BY `Days Since Install` 
        ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
    ) AS `Installs`
FROM your_table_name
ORDER BY `Days Since Install`;

PostgreSQL #

使用string_agg函数搭配窗口范围：

SELECT 
    "Days Since Install",
    string_agg("Installs", '+' ORDER BY "Days Since Install") OVER (
        ORDER BY "Days Since Install" 
        ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
    ) AS "Installs"
FROM your_table_name
ORDER BY "Days Since Install";

SQL Server（2017及以上版本） #

通过STRING_AGG函数实现：

SELECT 
    [Days Since Install],
    STRING_AGG([Installs], '+') WITHIN GROUP (ORDER BY [Days Since Install]) OVER (
        ORDER BY [Days Since Install] 
        ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING
    ) AS [Installs]
FROM your_table_name
ORDER BY [Days Since Install];

低版本数据库兼容方案（以MySQL 5.x为例） #

如果数据库不支持窗口函数，可以用自关联的方式实现：

SELECT 
    t1.`Days Since Install`,
    GROUP_CONCAT(t2.`Installs` SEPARATOR '+') AS `Installs`
FROM your_table_name t1
JOIN your_table_name t2 
    ON t2.`Days Since Install` >= t1.`Days Since Install`
GROUP BY t1.`Days Since Install`
ORDER BY t1.`Days Since Install`;

注意：请将上述语句中的your_table_name替换为实际的表名称。

内容的提问来源于stack exchange，提问作者Dogan Can Akca