Writing Nested SQL queries - sql

Question

I have 3 tables as StudentData which includes data of students, Subjects table which has data of all subjects offered and Marks which has the marks obtained by students for each subject.Marks table maps to StudentData table by StudentId and Subjects table by SubjectId
What I want to do is select the maximum marks for each subject and the name of the student like follows
So I wrote a Oracle PL/SQL Query as follows,
select MAX(marks)
from
(select Marks ,subjects.name as SJN ,studentdata.name
from (studentdata inner Join marks On studentdata.studentid = marks.studentid)
Inner Join subjects On subjects.subjectid = marks.subjectid)
where SJN in (select name from subjects);
But it gives only one result.Please help me to develop a query to get my expected result set.

Answer 1

Something like this? Lines #1 - 26 represent sample data (you don't type that); query you need begins at line #28.
SQL> with
2 -- sample data
3 studentdata (studentid, name, course) as
4 (select 1, 'olivier', 'it' from dual union all
5 select 2, 'noah', 'business' from dual union all
6 select 3, 'jack', 'business' from dual union all
7 select 4, 'mason', 'it' from dual union all
8 select 5, 'julion', 'it' from dual),
9 subjects (subjectid, name) as
10 (select 1, 'java' from dual union all
11 select 2, 'business stg' from dual union all
12 select 3, 'python' from dual union all
13 select 4, 'statistics' from dual union all
14 select 5, 'mgt accounting' from dual union all
15 select 7, 'social studies' from dual union all
16 select 8, 'ess english' from dual),
17 marks (id, studentid, subjectid, marks) as
18 (select 1, 1, 1, 56 from dual union all
19 select 2, 1, 2, 78 from dual union all
20 select 3, 1, 7, 83 from dual union all
21 select 4, 1, 3, 45 from dual union all
22 select 5, 1, 5, 63 from dual union all
23 --
24 select 6, 2, 1, 99 from dual union all
25 select 7, 3, 2, 10 from dual union all
26 select 8, 4, 7, 83 from dual)
27 --
28 select b.name subject, s.name student, m.marks
29 from marks m join subjects b on b.subjectid = m.subjectid
30 join studentdata s on s.studentid = m.studentid
31 where m.marks = (select max(m1.marks)
32 from marks m1
33 where m1.subjectid = m.subjectid
34 )
35 order by b.name, s.name;
SUBJECT STUDENT MARKS
-------------- ------- ----------
business stg olivier 78
java noah 99
mgt accounting olivier 63
python olivier 45
social studies mason 83
social studies olivier 83
6 rows selected.
SQL>

Answer 2

You can use the analytical function ROW_NUMBER as follows:
SELECT SJN, MARKS, STUNAME FROM
(SELECT
MARKS.MARKS,
SUBJECTS.NAME AS SJN,
STUDENTDATA.NAME AS STUNAME,
ROW_NUMBER() OVER (PARTITION BY SUBJECTS.SUBJECTID
ORDER BY MARKS.MARKS DESC NULLS LAST) AS RN
FROM
STUDENTDATA
INNER JOIN MARKS ON STUDENTDATA.STUDENTID = MARKS.STUDENTID
INNER JOIN SUBJECTS ON SUBJECTS.SUBJECTID = MARKS.SUBJECTID)
WHERE RN = 1;

Answer 3

The first thing that comes to mind: select the best mark per subject, then select the student(s) with that mark in that subject:
select s.name as subject, m.marks, sd.name as studentname
from marks m
join studentdata sd on sd.studentid = m.studentid
join subjects s on s.subjectid = m.subjectid
where (m.subjectid, m.marks) in
(
select subjectid, max(marks)
from marks
group by subjetid
);
As you see, we select twice from marks. This can be avoided with a window function:
select s.name as subject, m.marks, sd.name as studentname
from
(
select
subjectid,
marks,
max(marks) over (partition by subjectid) as max_marks
from marks
) m
join studentdata sd on sd.studentid = m.studentid
join subjects s on s.subjectid = m.subjectid
where m.marks = m.max_marks;
Another option is to join and to check that no better marks exist for the subject:
select s.name as subject, m.marks, sd.name as studentname
from marks m
join studentdata sd on sd.studentid = m.studentid
join subjects s on s.subjectid = m.subjectid
where not exists
(
select null
from marks m2
where m2.subjectid = m.subjectid
and m2.marks > m.marks
);
Which of these options is best, I cannot tell. Decide for the one that you find most readable. Regardless of which query you choose, this index should help the DBMS finding the highest marks quickly:
create index idx on marks(subjectid, marks);