Question

The following query is giving me differing results on different rows and it should be giving me the same result on all rows.

Here is the query:

select * from 
    (SELECT mjla_db.StudentRecordTable2.studentId, 
            mjla_db.StudentTable2.lastName as `Last Name`, 
            mjla_db.StudentTable2.firstName as `First Name`, 
            sum(if(quizId=60,quizGrade,0)) as `Quiz 60`, 
            sum(if(quizId=64,quizGrade,0)) as `Quiz 64`, 
            sum(if(quizId=71,quizGrade,0)) as `Quiz 71`,
            (sum(quizGrade*(1-abs(sign(quizId-60)))) + 
                sum(quizGrade*(1-abs(sign(quizId-64)))) + 
                sum(quizGrade*(1-abs(sign(quizId-71)))) )/3 
                as Averages 
    FROM mjla_db.StudentRecordTable2, mjla_db.StudentTable2 
    where (mjla_db.StudentRecordTable2.studentId=mjla_db.StudentTable2.studentId) 
    GROUP BY studentId) as A 
where A.studentId 
in (select mjla_db.ClassStudentTable2.studentId 
    from mjla_db.ClassStudentTable2 
    where mjla_db.ClassStudentTable2.classId='CS3071F2011');

The number of quizzes are dynamic, number of students are dynamic.

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Here is the contents of the table where mjla_db.StudentRecordTable2.classId='CS3071F2011'.

+-------------+-----------+-----------+--------+
| classId     | studentId | quizGrade | quizId |
+-------------+-----------+-----------+--------+
| CS3071F2011 | A1        |      NULL |     60 |
| CS3071F2011 | A2        |      NULL |     60 |
| CS3071F2011 | A5        |      NULL |     60 |
| CS3071F2011 | A1        |      NULL |     64 |
| CS3071F2011 | A2        |      NULL |     64 |
| CS3071F2011 | A5        |      NULL |     64 |
| CS3071F2011 | A7        |      NULL |     64 |
| CS3071F2011 | A3        |      NULL |     64 |
| CS3071F2011 | A4        |      NULL |     64 |
| CS3071F2011 | A3        |      NULL |     60 |
| CS3071F2011 | A4        |      NULL |     60 |
| CS3071F2011 | A7        |      NULL |     60 |
| CS3071F2011 | A1        |      NULL |     71 |
| CS3071F2011 | A2        |      NULL |     71 |
| CS3071F2011 | A5        |      NULL |     71 |
| CS3071F2011 | A7        |      NULL |     71 |
| CS3071F2011 | A3        |      NULL |     71 |
| CS3071F2011 | A4        |      NULL |     71 |
+-------------+-----------+-----------+--------+

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Here is the result of the query.

+-----------+-----------+------------+---------+---------+---------+----------+
| studentId | Last Name | First Name | Quiz 60 | Quiz 64 | Quiz 71 | Averages |
+-----------+-----------+------------+---------+---------+---------+----------+
| A1        | harry     | thomas     |       0 |       0 |       0 |   0.0000 |
| A2        | harry     | willy      |       0 |       0 |       0 |   0.0000 |
| A3        | billy     | gregory    |       0 |       0 |       0 |   0.0000 |
| A4        | goat      | bobb       |       0 |       0 |       0 |   0.0000 |
| A5        | nogood    | tom        |       0 |       0 |       0 |     NULL |
| A7        | foobar    | dick       |       0 |       0 |       0 |     NULL |
+-----------+-----------+------------+---------+---------+---------+----------+

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The expected result of the query would be for all the averages to be 0.0000, why are the last two varying?

Also, whenever i do the query so that i only get one student at a time A5 and A7 still show up as NULL and the rest show up fine…

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A solution to this problem or an alternate query to achieve the same goal would be acceptable. My goal is to get a table that each row is a representation of each student in the class and their grades on all the quizzes and an average of all the quizzes that student.

Answer 1

All of your quizGrade values are NULL, which you’re using in math calculations ((sum(quizGrade*(1-abs(sign(quizId-60)))). As soon as a db NULL enters into pretty much any kind of operation, the whole operation becomes null. The question shouldn’t be why the last two are null, but why the first four AREN’T.