PostgreSQL

Question

我有三张表。

Students
student_id | name
1            Rhon

Subjects
subject_id | subject_name | student_id
1            Physics        1
2            Math           1

Grades
grade_id | student_id | subject_id | grade
1          1            1            90
2          1            2            89
3          1            2            88

我希望结果是这样的：

student_id | student_name | subject_name | grades
1            Rhon           Physics        90
1            Rhon           Math           88,89

我当前的查询是：

SELECT students.student_id, subjects.subject_id, string_agg(grades.grade, ',')
FROM students
JOIN subjects ON students.student_id = subjects.student_id
JOIN grades ON subjects.subject_id = grades.subject_id;

我的查询有问题吗？我错过了什么吗？错误说 student_id 需要在 GROUP BY 子句中，但我不想那样。

Answer 1

您可以使用子查询来做到这一点：

SELECT s.student_id, s.student_name, j.subject_name, g.grades
FROM students s
JOIN subjects j
JOIN (
  SELECT student_id, subject_id, string_agg(grade, ',') AS grades
  FROM grades
  GROUP BY student_id, subject_id) g USING (student_id, subject_id);

你为什么不想 GROUP BY student_id？

PostgreSQL - 在 string_agg 加入

PostgreSQL - JOIN on string_agg

string-aggregation