表中每个记录的计数

Question

我有一张表格（stu_grades），用于存储他们参加过的中心的学生数据及其成绩

我想找出例如该表中的每个学生在任何中心获得“ A”然后“ B”等的次数

stu_grades

 stu_ID|grade1|grade2|Grade3|center
    1        A      A     C    1
    2        B      B     B    2
    3        C      C     A    1
    1        C      A     C    2

同一学生可能在表中出现多次相同的成绩，甚至相同的成绩，甚至不同的中心

我特别想检查成绩在哪里出现了3次或以上，以及它们存在多少个中心

因此，最终输出应为：

Stu_ID|Grade|Count|centercount
1       A      3       2 (As they accquired 'A' from 2 centres)
1       C      3       2 
2       B      3       1 (As they only exist in 1 centre)
3       C      2       1 
3       A      1       1

Answer 1

这是一种使用union all并集的方法将不同等级的数据分解为一列，然后进行汇总。

select stu_id,grade,count(*) cnt 
from (
select stu_id,grade_1 grade from stu_grades
union all
select stu_id,grade_2 grade from stu_grades
union all
select stu_id,grade_3 grade from stu_grades
) t
group by stu_id,grade

Answer 2

这应该满足您的要求：

SELECT [Stud_ID], Grade, count(*) as GradeCount FROM
  (SELECT [Stud_ID],Grade1 as Grade from [Stud_Details]
   UNION ALL
   SELECT [Stud_ID],Grade2 as Grade from [Stud_Details]
   UNION ALL
   SELECT [Stud_ID],Grade3 as Grade from [Stud_Details]) AS T
   GROUP BY T.[Stud_ID],T.Grade
   ORDER BY T.[Stud_ID]

更新：

您可以在GROUP BY之后使用HAVING来获取大于3的 gradeCount 。

SELECT [Stud_ID], Grade, count(*) as GrdCountStud FROM
  (SELECT [Stud_ID],Grade1 as Grade from [Stud_Details]
   UNION ALL
   SELECT [Stud_ID],Grade2 as Grade from [Stud_Details]
   UNION ALL
   SELECT [Stud_ID],Grade3 as Grade from [Stud_Details]) AS T
   GROUP BY T.[Stud_ID],T.Grade
     HAVING COUNT(*) > 3   
   ORDER BY T.[Stud_ID]