根据列值创建列

Question

我不会与 SQL 一起工作。我被要求创建一个 table 来显示学生在课程管理系统中每个月可以参加的不同课程的能力期望的结果是如下所示的输出：

month       |  math | english
------------+-------+------------
12/1/2014   | 42    | 137   
1/1/2015    | 137   | 76    
2/1/2015    | 139   | 79    
3/1/2015    | 143   | 83

"Math"和"English"是course_kinds，course_kind是tablecourse_capacities上的一列。是否可以根据列中的各种值创建列？

我当前的 SQL 产生了这个结果，这不是最佳的：

 course_kind |   month    | capacity
-------------+------------+----------
 math         | 2015-01-01 |       47
 math         | 2015-02-01 |       43
 math         | 2015-03-01 |       43
 math         | 2015-04-01 |       45
 math         | 2015-05-01 |       46
 math         | 2015-06-01 |       46
 math         | 2015-07-01 |       46
 math         | 2015-08-01 |       46
 math         | 2015-09-01 |       47
 math         | 2015-10-01 |       47
 math         | 2015-11-01 |       47
 math         | 2015-12-01 |       47
 english      | 2015-01-01 |       97
 english      | 2015-01-01 |       73
 english      | 2015-02-01 |       76
 english      | 2015-03-01 |       79
 english      | 2015-04-01 |       83
 english      | 2015-05-01 |       83
 english      | 2015-06-01 |       87
 english      | 2015-07-01 |       89
 english      | 2015-08-01 |       91
 english      | 2015-09-01 |       93
 english      | 2015-10-01 |       97
 english      | 2015-11-01 |       97
 english      | 2015-12-01 |       97

这是SQL：

SELECT
    course_capacities.course_kind,
    monthly_course_capacities.month,
    monthly_course_capacities.capacity
FROM course_capacities
    INNER JOIN monthly_course_capacities ON course_capacities.id = monthly_course_capacities.course_capacity_id
    ORDER BY course_capacities.course_kind ASC,
    monthly_course_capacities.month ASC;

Answer 1

您可以使用 case based aggregation，以防 course_kind 列值已知且有限，否则，需要创建动态查询。

SELECT
    monthly_course_capacities.month,
    max(case when course_capacities.course_kind ='math' then capacity end ) as math,
    max(case when course_capacities.course_kind ='english' then capacity end ) as english,
FROM course_capacities
INNER JOIN monthly_course_capacities ON course_capacities.id = monthly_course_capacities.course_capacity_id
GROUP BY monthly_course_capacities.month

根据列值创建列

creating columns based on column values

sql

postgresql

pivot