Question

我不太强烈地使用SQL。我被要求创建一个表格，显示学生在课程管理系统中每个月可以采取的不同课程的能力所需的结果是输出如下：

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

＆＃34;数学＆＃34;和＆＃34;英语＆＃34;是course_kind s，course_kind是表course_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产生的结果不是最佳结果：

 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run_kind列值已知并且受限制，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

1 个答案: