3

我有一个名为 STUDENTS 的 APACHE DERBY 数据库表，其中包含以下行：

_____________________________
| NAME  |COUNTER|   MONTH   |
-----------------------------
| bill  |   10  |   January |
-----------------------------
| bill  |   12  |   February|
-----------------------------
| bill  |   11  |   March   |
-----------------------------
| sam   |   14  |   January |
-----------------------------
| sam   |   9   |   February|
-----------------------------
| sam   |   22  |   March   |
-----------------------------
| hal   |   21  |   January |
-----------------------------
| hal   |   18  |   February|
-----------------------------
| hal   |   19  |   March   |
-----------------------------

我正在尝试找到获得以下输出的语法...

_________________________________
| NAME  |  JAN  |  FEB  |   MAR |
---------------------------------
| bill  |  10   |  12   |  11   |
---------------------------------
| sam   |  14   |   9   |  22   |
---------------------------------
| hal   |  21   |  18   |  19   |
---------------------------------

我尝试了以下方法：

select distinct(name), 
    (SELECT COUNTER FROM STUDENTS WHERE NAME = 'bill' AND month = 'january') as jan,
    (SELECT COUNTER FROM STUDENTS WHERE NAME = 'bill' AND month = 'february') as feb,
    (SELECT COUNTER FROM STUDENTS WHERE NAME = 'bill' AND month = 'march') as mar
    FROM STUDENT where name = 'bill'

......它有点像一个学生的概念证明。但是，它很冗长，仅适用于一名学生，并且不适用于我将添加到数据库中的未来几个月的任何数据。这意味着，无论数据库中存在多少个月的数据，我都希望能够获得每个 NAME 的最后 3 个 COUNTER 值。

我查看了多个与嵌套查询相关的 so 问题，但没有一个有帮助。

建议？

任何帮助表示赞赏！

4

1 回答 1

5

您可以使用条件聚合

SELECT name,
       Max(CASE WHEN month = 'January'  THEN counter end) AS jan,
       Max(CASE WHEN month = 'February' THEN counter end) AS feb,
       Max(CASE WHEN month = 'March'    THEN counter end) AS mar
  FROM student
 GROUP BY name
于 2019-03-23T23:10:22.840 回答