Question

我有三张桌子，如下所示。

TABLE1 : tb_subject
subject_id  subject_name
  1          English
  2          Maths
  3          Science

Table2 : tb_student
subject_id  student_id 
    1         AA        
    1         BB
    2         CC
    3         DD
    3         EE

  Table3 : tb_student_score
   student_id  score  conducted_month_number
      AA         20          2
      BB         30          3
      CC         50          4
      AA         80          4
      DD         50          6
      BB         10          2
      EE         40          3

结果应为

  conducted_month_number SUM(subject_id1)  SUM(subject_id2)  SUM(subject_id3) 
         1                   0                  0                   0
         2                  30                  0                   0
         3                  30                  0                  40
         4                  80                 50                   0
         5                   0                  0                   0
         6                   0                  0                  60
         7                   0                  0                   0
         8                   0                  0                   0
         9                   0                  0                   0
         10                  0                  0                   0
         11                  0                  0                   0
         12                  0                  0                   0

如何为此编写选择查询？可以在结果输出中添加未存储在表中的所有月份编号吗？

Answer 1

您应该可以使用case when分别为每个主题求和：

SELECT conducted_month_number,
       SUM(CASE b.subject_id WHEN 1 THEN a.score ELSE 0 END) AS English,
       SUM(CASE b.subject_id WHEN 2 THEN a.score ELSE 0 END) AS Maths,
       SUM(CASE b.subject_id WHEN 3 THEN a.score ELSE 0 END) AS Science
FROM tb_student_score AS a
  JOIN tb_student AS b ON b.student_id = a.student_id
GROUP BY conducted_month_number
ORDER BY conducted_month_number;

然而，单凭这一点并不能确保您获得的conducted_month_number值不存在 - 如果这是一个问题，您可以创建一个每月得分为0的虚拟学生

编辑：我注意到在我提交答案的同时发布了一些评论 - 如果你想根据tb_subject表中行的值来改变求和列的数量，你将找不到{的关系模型{1}}非常适合这项任务。但是，您可以轻松返回并更新您的查询，以包含您稍后可能添加的任何新主题。

Answer 2

使用SQL添加了1到12个月的dummy values，之后又添加了union statement来计算总分。

试试这个： -

group by

查询将三个表连接为行值作为列标题

2 个答案: