Question

如何仅对SQL查询中的学分总和大于或等于15个学分的那些学生获得学分总和的平均值？

使用HAVING（SUM（STC_ATT_CRED））> = 15.0时，它为我提供了所有学生的平均数，而不仅仅是具有15.0或更高总学分的学生。

SELECT (SUM(STC_ATT_CRED)  )/COUNT(DISTINCT stc_person_id) as 'average attempted credit',
COUNT (DISTINCT stc_person_id)         
FROM  dbo.S85_STUDENT_ACAD_CRED  
WHERE (STC_TERM='2018FA') AND ((STC_VERIFIED_GRADE IS NOT NULL ))   
AND STC_PERSON_ID IN (75 student id's in here)
HAVING (SUM(stc_att_cred)) >= 15.0

我希望输出为仅符合条件的46名学生的尝试学分总数的平均值（尝试学分总数15或更多）。

取而代之的是，输出结果使我平均获得了全部75名学生，而没有将其范围缩小到只有15个或更多学分的学生。

Answer 1

首先GROUP BY stc_person_id得到每个学生的总和，然后以所有学生的stc_att_cred >= 15.0的总和为条件来过滤结果，并得到平均值：

SELECT 
  SUM(t.studentsum) / COUNT(*) 'average attempted credit',
  count(*)
FROM (
  SELECT 
    stc_person_id, 
    SUM(STC_ATT_CRED) studentsum
  FROM dbo.S85_STUDENT_ACAD_CRED
  WHERE 
    STC_TERM='2018FA'
    AND 
    STC_VERIFIED_GRADE IS NOT NULL
    AND 
    STC_PERSON_ID IN (75 student id's in here)
  GROUP BY stc_person_id
) t
WHERE t.studentsum >= 15.0

Answer 2

我会用一个内部表来做到这一点：

SELECT avg here
FROM
( 
    --Get the students and sum credits
    --Filter this list for students with sum >= 15
    SELECT STC_PERSON_ID, SUM(stc_att_cred) as studentSum     
    FROM  dbo.S85_STUDENT_ACAD_CRED  
    WHERE (STC_TERM='2018FA') AND ((STC_VERIFIED_GRADE IS NOT NULL ))  AND 
           STC_PERSON_ID IN (75 student id's in here)
    GROUP BY STC_PERSON_ID
    HAVING (SUM(stc_att_cred)) >= 15.0
)studentsWithSumCreditsOverFifteen

如何仅对总和大于SQL查询中的那些求平均

2 个答案: