SQL: 如何 select 共享 NAME 字段的多个 COUNT 值？

Question

我有一个名为 STUDENTS 的 APACHE DERBY 数据库 table，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。然而，它很冗长，它只适用于一个学生，而且它不适用于来自我将添加到数据库的未来几个月。意思是，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名称的最后 3 个计数器值。

我看过多个 s.o。与嵌套查询相关的问题，但其中 none 个问题有所帮助。

建议？

感谢任何帮助！

Answer 1

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

SQL: 如何 select 共享 NAME 字段的多个 COUNT 值？

SQL: How to select multiple COUNT values that share NAME field?

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