Database Technology - postgresql - sql

Question

I am quite new to postgresql. Could any expert help me solve this problem please.
Consider the following PostgreSQL tables created for a university system recording which students take which modules:
CREATE TABLE module (id bigserial, name text);
CREATE TABLE student (id bigserial, name text);
CREATE TABLE takes (student_id bigint, module bigint);
Rewrite the SQL to include sensible primary keys.
CREATE TABLE module
(
m_id bigserial,
name text,
CONSTRAINT m_key PRIMARY KEY (m_id)
);
CREATE TABLE student
(
s_id bigserial,
name text
CONSTRAINT s_key PRIMARY KEY (s_id)
);
CREATE TABLE takes
(
student_id bigint,
module bigint,
CONSTRAINT t_key PRIMARY KEY (student_id)
);
Given this schema I have the following questions:
Write an SQL query to count how many students are taking DATABASE.
SELECT COUNT(name)
FROM student
WHERE module = 'DATABASE' AND student_id=s_id
Write an SQL query to show the student IDs and names (but nothing else) of all student taking DATABASE
SELECT s_id, name
FROM Student, take
WHERE module = 'DATABASE' AND student_id = s_id
Write an SQL query to show the student IDs and names (but nothing else) of all students not taking DATABASE.
SELECT s_id, name
FROM Student, take
WHERE student_id = s_id AND module != 'DATABASE'
Above are my answers. Please correct me if I am wrong and please comment the reason. Thank you for your expertise.

Answer 1

This looks like homework so I'm not going to give a detailed answer. A few hints:
I found one case where you used ´ quotes instead of ' apostrophes. This suggests you're writing SQL in something like Microsoft Word, which does so-called "smart quotes. Don't do that. Use a sensible text editor. If you're on Windows, Notepad++ is a popular choice. (Fixed it when reformatting the question, but wanted to mention it.)
Don't use the legacy non-ANSI join syntax JOIN table1, table2, table3 WHERE .... It's horrible to read and it's much easier to make mistakes with. You should never have been taught it in the first place. Also, qualify your columns - take.module not just module. Always write ANSI joins, e.g. in your example above:
FROM Student, take
WHERE module = 'DATABASE' AND student_id = s_id
becomes
FROM student
INNER JOIN take
ON take.module = 'DATABASE'
AND take.student_id = student.s_id;
(if the table names are long you can use aliases like FROM student s then s.s_id)
Query 3 is totally wrong. Imagine if take has two rows for a student, one where the student is taking database and one where they're taking cooking. Your query will still return a result for them, even though they're taking database. (It'd also return the same student ID multiple times, which you don't want). Think about subqueries. You will need to query the student table, using a NOT EXISTS (SELECT .... FROM take ...) to filter out students who are not taking database. The rest you get to figure out on your own.
Also, your schemas don't actually enforce the constraint that a student may only take DATABASE once at a time. Either add that, or consider in your queries the possibility that a student might be registered for DATABASE twice.