如何计算 PostgreSQL 中列的每个不同值的出现次数？
[英]How can I do count on number of occurrences of each distinct value of a column in PostgreSQL?
问题描述
List of students enrolled in courses.
参加课程的学生名单。 In each course there could be x homeworks.每门课程可能有 x 个作业。 I need count of homeworks for each course for each student.我需要为每个学生计算每门课程的作业。
Eg: Sample table例如：样品表
Mark Math hw1
David. Math hw2
Mark. Science. hw1
Mark Math. hw2
David. Science. hw1
David. Literature. hw1
Expected results (3 columns)
Mark | Math | 2 (because two math homeworks)
Mark | Science | 1 home work
David| Math | 1 home work
David| Science | 1 home work
David| Lit. | 1 home work
1 个解决方案
解决方案1
1 已采纳 2020-08-14 00:24:51
select name, course, count(*)
from your_table
group by name, course
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