Select three different things from three diffrent tables - mysql

Question

I wanna a query to get first_name of students and first_name of teachers which have the most courses with each other with the number of these courses.
Table Student:
CREATE TABLE Student(
id INT AUTO_INCREMENT,
first_name VARCHAR(255) NOT NULL,
last_name VARCHAR(255),
email VARCHAR(255) UNIQUE,
PRIMARY KEY (id)
);
Table Teacher:
CREATE TABLE Teacher(
id INT AUTO_INCREMENT,
first_name VARCHAR(255) NOT NULL,
last_name VARCHAR(255),
email VARCHAR(255) UNIQUE,
degree VARCHAR(10) NUT NULL,
PRIMARY KEY (id)
);
Table Course:
CREATE TABLE Course(
id INT AUTO_INCREMENT,
code INT NOT NULL UNIQUE,
name VARCHAR(255) NOT NULL,
st_id INT,
teach_id INT,
PRIMARY KEY (id),
FOREIGN KEY st_id REFERENCES Student (id),
FOREIGN KEY teach_id REFERENCES Teacher (id)
);
Is the below query correct? i.e. Can I use 3 SELECT in a query?
query1:
SELECT S.first_name
FROM Student AS S
INNER JOIN Course AS C
ON C.st_id = S.id
SELECT T.first_name
FROM Teacher AS T
INNER JOIN Course AS CC
ON CC.teach_id = T.id
SELECT COUNT(*)
FROM Course
WHERE Course.st_id = S.id
AND Course.teach_id = T.id
GROUP BY COUNT(*)
ORDER BY DESC;
query2:
SELECT S.first_name, T.first_name, COUNT(*)
FROM Student AS S, Teacher AS T, Course
WHERE Course.st_id = S.id
AND Course.teach_id = T.id
GROUP BY COUNT(*)
ORDER BY DESC;
If the above queries are not correct(probably the first one is wrong) guide me to correct answer, please.
NOTE: If the ordering isn't unique, order by the name of teachers first, then order by the name of the students(for clarity but not important so much to me).

Answer 1

Your second query is closer to being right but it has some issues. I would recommend using JOIN statements rather than implied joins. This makes the query easier to read.
Something like this should work:
SELECT t.first_name,
t.id,
s.first_name,
s.id,
COUNT(*) AS course_count
FROM Course c
JOIN Student s ON c.st_id = s.id
JOIN Teacher t ON c.teach_id = t.id
GROUP BY t.id, s.id
ORDER BY course_count DESC, t.first_name, s.first_name;
You need to add a group by in order to get your count on a per student basis. Putting the group by on the id columns rather than the name makes sure you get counts on unique students and teachers in case you have multiple records in your table with the same first name. I am also adding the id columns to the select for the same reason, but these are not necessary and can be removed without affecting the accuracy of the query.

Answer 2

SELECT t.first_name, t.id, s.first_name, s.id, COUNT(c.id) AS course_count
FROM course c
JOIN student s ON c.st_id = s.id
JOIN teacher t ON c.teach_id = t.id
GROUP BY t.id, s.id
ORDER BY t.first_name, s.first_name
The essential data is contained in the Courses table, with the Student and Teacher tables only required for gathering the names. This query joins the 3 tables in question, computing the count of courses shared by teachers and students.