sql – 具有列标题的列名和基于其他表中的列标签的count / Pivot

2019年8月4日 178次阅读

我有两张表如下：

TABLE标记列表：

student_id  class_id    subject_1   subject_2   subject_3   subject_4   subject_5
----------- ----------- ----------- ----------- ----------- ----------- ----------
1           9           78          87                                  95
2           9           67          95                                    87
3           9           85          84                                    85
4           10          70                      65          78    
5           10          75                      80          81    
6           10          80                      75          82

表subject_names

column_name     subject_name
--------------- -------------
subject_1       English
subject_2       Chemistry
subject_3       Economics
subject_4       Accounts
subject_5       Biology

现在,我需要为class_id = 9生成这样的报告

column_name     subject_name  no_of_students
--------------- ------------- --------------
subject_1       English       3
subject_2       Chemistry     3
subject_3       Economics     0
subject_4       Accounts      0
subject_5       Biology       3

简而言之,我必须使用column_names,subject_name和class_id = 9(或10,无论如何)为该主题出现的学生数生成一个报告.

我所能做的就是

1.

    SELECT sn.column_name, sn.subject_name FROM subject_names sn;

和

2.

    SELECT ml.class_id,
           count(ml.subject_1) AS s1,
           count(ml.subject_2) AS s1,
           count(ml.subject_3) AS s1,
           count(ml.subject_4) AS s1,
           count(ml.subject_5) AS s1,
      FROM marklist ml
     WHERE ml.class_id = 9;

我不明白如何继续使用查询1来转动查询2的结果.我可能会走错方向,但我不知道.

最佳答案 你可以在
unpivot你的marklist表和外连接它与subject_names表.

with unpivot_x(student_id,class_id,subject_code,marks) as (
    select * from marklist
    unpivot (marks for subject_code in (  subject_1 as 'subject_1',
                                          subject_2 as 'subject_2',
                                          subject_3 as 'subject_3',
                                          subject_4 as 'subject_4',
                                          subject_5 as 'subject_5'
                                      )
                                      ))
select a.column_name,a.subject_name, count(b.student_id)
  from subject_names a left outer join unpivot_x b
    on a.column_name = b.subject_code and b.class_id = 9
 group by a.column_name,a.subject_name
 order by 1;

演示于sqlfiddle.

点赞