SQL query for extracting data from related tables - mysql

Question

Although I found several similar problems but I couldn't apply them to resolve my issue.
Problem statement:- I have three MySql tables STUDENT,SUBJECT and STUDIES as described below:-
STUDENT-
rollno Name
X1 Alpha
X2 Beta
Y1 Zeta
X3 Omega
here the alphabet in each rollno corresponds to student in same batch/class. E.g students X1,X2 and X3 belong to same class whereas Y1 is that of a different one.
SUBJECT-
Code Title Credits
abc subject1 2
bcd subject2 4
gfp subject3 3
STUDIES-
rollno code
X1 abc
X1 bcd
X1 gfp
X2 bcd
X2 abc
Y1 gfp
X3 abc
I need help in framing mysql queries for:
a) displaying the credits undertaken by each student.
Like
Rollno Name Credits
X1 Alpha 9
X2 Beta 6
Y1 Zeta 3
X3 Omega 2
The best that I have come up is with this
select rollno,
(select sum(credits) from subject
where studies.code=subject.code)
from studies;
But what I get are the rollno and credits displayed individually for every subject a student studies.(I haven't been able to scale my query to get the name of student from the third table yet)
b) Finding out the subjects which have been taken by all the students of a class/batch.
In the given scenario the answer would be
Batch Subject Code Title
X abc subject1
I can extract out distinct batches by string processing but dont know how to proceed further from that point.
c) Being a MySql newbie, could you also point me out to some good web resource with practice problems for learning advanced queries like these. I have gone through a few of them on the last couple days but have not found them sufficient in developing concepts required to get through my queries.
EDIT: Sharing the CREATE queries for the tables:-
For SUBJECT:
CREATE TABLE IF NOT EXISTS subject (
code varchar(8) UNIQUE, title varchar(75) NOT NULL,
credits int, check (credits <5),PRIMARY KEY (code));
For STUDENT:
CREATE TABLE IF NOT EXISTS student (
rollno varchar(9) UNIQUE,name varchar(50));
For Studies:
CREATE TABLE IF NOT EXISTS studies (
rollno varchar(9) NOT NULL,code varchar(50) NOT NULL,
FOREIGN KEY(rollno) REFERENCES student(rollno));

Answer 1

Well, for your first query, you want to join across all three tables, looking for rollno, name, and credits. So at a first pass, you need to join like this:
SELECT s.rollno, s.name, sb.credits
FROM student s
INNER JOIN studies st
ON st.rollno = s.rollno
INNER JOIN subject sb
ON sb.code = st.code
This is part of the solution - it gives you the information you want, and now you just have to use an aggregate function to tally up the credits, using SUM and GROUP BY:
SELECT s.rollno, s.name, SUM(sb.credits) AS credits
FROM student s
INNER JOIN studies st
ON st.rollno = s.rollno
INNER JOIN subject sb
ON sb.code = st.code
GROUP BY s.rollno, s.name
The second part is tougher, and there are likely other (and better) ways to do this, but here's my approach:
SELECT q1.batch, q1.code, sb.title
FROM
(SELECT st.code, SUBSTR(st.rollno,1,1) batch,
COUNT(SUBSTR(st.rollno,1,1)) numb
FROM studies st
GROUP BY st.code, SUBSTR(st.rollno,1,1)) q1
INNER JOIN
(SELECT SUBSTR(s.rollno,1,1) batch,
COUNT(SUBSTR(s.rollno,1,1)) numb
FROM student s
GROUP BY SUBSTR(s.rollno,1,1)) q2
ON q1.batch = q2.batch AND q1.numb = q2.numb
INNER JOIN subject sb
ON q1.code = sb.code
Some explanation: the first sub-query (q1) counts the number of students from each batch in each subject. The output from that would be:
abc x 3
bcd x 2
gfp x 1
gfp y 1
The second subquery (q2) counts the number of students in each batch, with output:
x 3
y 1
By JOINing these two subqueries, we select only those subjects where the batch and the batch count are the same:
abc x 3
gfp y 1
Finally, JOIN on the subject table to get the subject title included (and set the starting SELECT statement to only select batch, code, and title), giving output:
x abc subj1
y gfp subj3
Note that the last row here (y - subj3) is valid, since every member of the 'y' batch (of which there is only one) is enrolled in course gfp.
As for recommended sites and resources - that's a bit outside the scope of SO. You can find lots of good online resources by Googling 'SQL tutorial' or 'SQL online courses'. Lots of good free stuff out there.