Connecting 4 tables - sql

Question

I am only a beginner in SQL, and I have problem that I can not solve.
The problem is the following:
i have four tables
Student: matrnr, name, semester, start_date
Listening: matrnr<Student>, vorlnr<Subject>
Subject: vorlnr, title, sws, teacher<Professor>
Professor: persnr, name, rank, room
I need to list all the students that are listening the Subject of some Professor with samo name.
EDIT:
select s.*
from Student s, Listening h
where s.matrnr=h.matrnr
and h.vorlnr in (select v.vorlnr from Subject v, Professor p
where v.gelesenvon=p.persnr and p.name='Kant');
This is how i solved it but i am not sure is it optimal solution.

Answer 1

Your approach is good. Only, you want to show students, but join students with listings thus getting student-listing combinations.
Moreover you use a join syntax that is out-dated. It was replaced more than twenty years ago with explicit joins (INNER JOIN, CROSS JOIN, etc.)
You can do it with subqueries only:
select *
from Students,
where matrnr in
(
select matrnr
from Listening
where vorlnr in
(
select vorlnr
from Subject
where gelesenvon in
(
select persnr
from Professor
where name='Kant'
)
)
);
Or join the other tables:
select *
from Students
where matrnr in
(
select l.matrnr
from Listening l
inner join Subject s on s.vorlnr = l.vorlnr
inner join Professor p on p.persnr = s.gelesenvon and p.name='Kant'
);
Or with EXISTS:
select *
from Students s
where exists
(
select *
from Listening l
inner join Subject su on su.vorlnr = l.vorlnr
inner join Professor p on p.persnr = su.gelesenvon and p.name='Kant'
where l.matrnr = s.matrnr
);
Some people like to join everthing and then clean up in the end using DISTINCT. This is easy to write, especially as you don't have to think your query through at first. But for the same reason it can get complicated when more tables and more logic are involved (like aggregations) and it can become quite hard to read, too.
select distinct s.*
from Students s
inner join Listening l on l.matrnr = s.matrnr
inner join Subject su on su.vorlnr = l.vorlnr
inner join Professor p on p.persnr = su.gelesenvon and p.name='Kant';
At last it is a matter of taste.

Answer 2

When you have an SQL problem, a good way of presenting the problem is to show us the tables as CREATE TABLE statements. Such statements show details such as the types of the columns and which columns are primary keys. Additionally this allows us to actually build a little database in order to reproduce a faulty behavior or just to test our solutions.
CREATE TABLE Student
(
matrnr NUMBER(9) PRIMARY KEY,
name NVARCHAR2(50),
semester NUMBER(2),
start_date DATE
);
CREATE TABLE Listening
(
matrnr NUMBER(9), -- Student
vorlnr NUMBER(9), -- Subject
CONSTRAINT PK_Listening PRIMARY KEY (matrnr, vorlnr)
);
CREATE TABLE Subject
(
vorlnr NUMBER(9) PRIMARY KEY,
title NVARCHAR2(50),
sws NVARCHAR2(50),
teacher NUMBER(9) -- Professor
);
CREATE TABLE Professor
(
persnr NUMBER(9) PRIMARY KEY,
name NVARCHAR2(50),
rank NUMBER(3),
room NVARCHAR2(50)
);
Using this schema, my solution would look like this:
SELECT *
FROM
Student
WHERE
matrnr IN (
SELECT L.matrnr
FROM
Listening L
INNER JOIN Subject S
ON L.vorlnr = S.vorlnr
INNER JOIN Professor P
ON S.teacher = P.persnr
WHERE P.name = 'Kant'
);
You can find it here: http://sqlfiddle.com/#!4/5179dc/2
Since I didn't insert any records, the only thing it is testing is the syntax and the correct use of table and column names.
Your solution is suboptimal. It does not differentiate between joining of tables and additional conditions specified as where-clause. It can produce several result records per student if they attend several courses of the professor. Therefore my solution puts all the other tables into the sub-select.

Answer 3

select st.name
from student st
join listening l on l.matrnr = st.matrnr
join subject su on su.vorlnr = l.vorlnr
join professor p on su.teacher = p.persnr
where p.name = 'some name'

Answer 4

SELECT *
FROM student
INNER JOIN listening ON student.matrnr = listening.matrnr
INNER JOIN subject ON listening.vorlnr = subject.vorlnr
INNER JOIN professor ON subject.teacher = professor.name
WHERE professor.name = 'some name'