I have a table (stu_grades) that stores student data and their grades.
I want to find out how many times for eg each student in that table got 'A' and then 'B' etc.
How do i do this? my feeling is to use a row over? by i can't construct the query.
stu_grades
stu_ID|grade1|grade2|Grade3
1 A A C
2 B B B
3 C C A
1 C A C
the same student could occur more than once in the table with the same grades or even a different grade.
I especially want to check where the grade has appeared more than 3 or more times
So the final output should be like:
Stu_ID|Grade|Count
1 A 3
1 C 3
2 B 3
3 C 2
3 A 1
Use the Group BY
:
SELECT stu_ID, Grade, GradeCount
FROM stu_grades
LEFT OUTER JOIN
(
SELECT Grade, COUNT(Grade) GradeCount
FROM stu_grades
GROUP BY Grade
)tb
ON tb.Grade= stu_grades.Grade
您可以使用GROUP BY和HAVING子句解决此问题
SELECT grade, COUNT(grade) FROM stu_grades GROUP BY grade HAVING COUNT(grade) > 3
You mentioned that you wanted the grade count per student per grade. You would need to group by both stu_id and grade.
SELECT stu_id, grade, COUNT(grade) GradeCount
FROM stu_grades
GROUP BY stu_id, grade
HAVING COUNT(grade) > 3
1) Normalize your data:
select stu_id, grade
from stu_grades, unnest(array[grade1, grade2, grade3]) as g(grade);
2) For now it is easy:
select stu_id, grade, count(*)
from stu_grades, unnest(array[grade1, grade2, grade3]) as g(grade)
group by 1, 2 order by 1, 2;
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