MySQL和PHP：Sum（marks）以及where子句

Question

I have been trying to make a query to show the rows in the table that correspond to my query and get their results but it is not working as it shows only one row. My query is:

SELECT *, sum(marks) 
FROM studentresult, subject 
where studentresult.term='1' 
  and studentresult.studentid='2' 
  and studentresult.year='2015' 
  and studentresult.subjectid = subject.id

It should be like this:

Subjectid | Studentid | Marks | Term | Year
1         | 2         | 99    | 1    | 2015
1         | 2         | 2     | 1    | 2015
                        101

But it is showing like this:

Subjectid | Studentid | Marks | Term | Year | Sum(mark)
1         | 2         | 99    | 1    | 2015 | 101

Can someone help me?
Thanking you in advance

rgds
Akshat

Answer 1

SELECT subjectId, studentID, marks, term, Year, sum(marks)
FROM studentresult SR
 LEFT JOIN subject S 
where sr.term='1' 
  and sr.studentid='2' 
  and sr.year='2015' 
  and sr.subjectid = subject.id
Group by subjectId, studentID, marks, term, Year

I'm not sure what tables have what columns, so you'll need to adjust this... but you need a group by to get the multiple rows or mysql group by extension will randomly select values in the non-aggregated columns. Generally you need to group by all the non-aggregated columns in the select provided they would have different values. If they have the same value, then mysql group by extension can handle it and they are not needed in the group by.

Logically in this case, I think all values in your expected result should be in the group by as you may have the same person, over multiple years and terms. Mark is the only value which May be able to be excluded.

Now that I see you want a sum of marks though... we'll have to do a sub-select to get that value as mySQL doesn't have the windowed sets logic. using over syntax... This windowed set logic allows you do do inline aggregation without effecting the results of the main query. Since mySQL doesn't support it, we use a subselect.

While I switched to using the INNER JOIN syntax vs the , notation, both would work; I'm just more comfortable with the Inner join syntax.

SELECT subjectId, studentID, marks, term, Year, sMarks
FROM studentresult SR
INNER JOIN subject S 
  on  SR.subjectid = s.id
INNER JOIN (SELECT sum(Marks) smarks, Year, Term, StudentID, Subject
            FROM studentResult
            GROUP BY Year, Term, StudentID, Subject) Sub
   ON SR.studentID =Sub.StudentID
  and SR.Subject = Sub.Subject
  and SR.Year = Sub.Year
  and SR.Term = Sub.Term
WHERE SR.term='1' 
  and SR.studentid='2' 
  and SR.year='2015' 

Answer 2

I know you already accepted an answer but just to let you know that, in fact, you can achieve exactly what you want. Also, since you don't retrieve any data from subject table, you can remove the join to it.

SQL Fiddle

MySQL 5.5.32 Schema Setup :

CREATE TABLE studentresult
    (`Subjectid` int, `Studentid` int, `Marks` int, `Term` int, `Year` int)
;

INSERT INTO studentresult
    (`Subjectid`, `Studentid`, `Marks`, `Term`, `Year`)
VALUES
    (1, 2, 99, 1, 2015),
    (1, 2, 2, 1, 2015)
;

CREATE TABLE subject
    (`id` int, `Subject` varchar(8))
;

INSERT INTO subject
    (`id`, `Subject`)
VALUES
    (1, 'Whatever')
;

Query 1 :

SELECT subjectId, studentID, marks, term, Year
FROM studentresult
INNER JOIN subject ON studentresult.subjectid = subject.id
WHERE term='1' 
  and studentid='2' 
  and year='2015' 
UNION
SELECT '', '', sum(marks), '', ''
FROM studentresult 
INNER JOIN subject ON studentresult.subjectid = subject.id
WHERE term='1' 
  and studentid='2' 
  and year='2015' 
GROUP BY subjectId, studentID, term, Year
ORDER BY marks

Results :

| SUBJECTID | STUDENTID | MARKS | TERM | YEAR |
|-----------|-----------|-------|------|------|
|         1 |         2 |     2 |    1 | 2015 |
|         1 |         2 |    99 |    1 | 2015 |
|           |           |   101 |      |      |

Answer 3

I have to tell you sir that you cannot append a custom row after your query. The tables (query results) are joined. The result is showing correctly. you need to divide the query into 2 sub queries:

SELECT *
FROM studentresult, subject 
where studentresult.term='1' 
  and studentresult.studentid='2' 
  and studentresult.year='2015' 
  and studentresult.subjectid = subject.id

And to get the sum u can have:

SELECT sum(marks) 
FROM studentresult, subject 
where studentresult.term='1' 
  and studentresult.studentid='2' 
  and studentresult.year='2015' 
  and studentresult.subjectid = subject.id

It is the better way to do it. Just so you know it is not always efficient to join your queries...