Choosing applicants for positions via a MySQL query - mysql

Question

I have the following MySQL tables. They represent CS courses at a school and applicants to be a TA (teaching assistant) for specific courses.
I want to create a query that will print the "best" applicant for each course. The constraints for the best applicant are:
Applicants with Applicants.level = 7 are matched first.
Applicants with ApplicantsToCourses.returning = true are chosen second.
All other applicants are matched without further discrimination.
The table definitions are:
CREATE TABLE Courses (
course_number SMALLINT(3) UNSIGNED NOT NULL,
course_section SMALLINT(1) UNSIGNED NOT NULL,
name CHAR(30) NOT NULL,
instructor CHAR(30),
lab_time CHAR(30),
PRIMARY KEY(course_number, section),
FOREIGN KEY(course_number, section) REFERENCES ApplicantsToCourses(course_number, course_section)
)
CREATE TABLE Applicants (
student_id CHAR(10) NOT NULL,
name CHAR(30),
email CHAR(30),
gpa DECIMAL(4,3) UNSIGNED,
level CHAR(2),
timestamp TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY(student_id),
FOREIGN KEY(student_id) REFERENCES ApplicantsToCourses(student_id),
CHECK(gpa <= 4.000)
)
CREATE TABLE ApplicantsToCourses (
student_id CHAR(10) NOT NULL,
returning BOOLEAN DEFAULT FALSE NOT NULL,
course_number SMALLINT(3) UNSIGNED NOT NULL,
course_section SMALLINT(1) UNSIGNED NOT NULL,
PRIMARY KEY(student_id, course_number, course_section),
FOREIGN KEY(student_id) REFERENCES Applicants(student_id),
FOREIGN KEY(course_number, course_section) REFERENCES Courses(course_number, course_section)
)
My attempt at a query was . . .
select a.student_id, ac.course_number, ac.course_section
from Applicants a, ApplicantsToCourses ac, Courses c
where a.student_id = ac.student_id and ac.course_number = c.course_number and ac.course_section = c.course_section
order by a.level, ac.returning desc
. . . but that certainly doesn't have the correct logic.

Answer 1

You can use the following pseudo code to create some temporary tables that should help you reach your final solution.
SELECT *
FROM Applicants APP
JOIN ApplicantsToCourses ATC ON ATC.student_id = APP.student_id
JOIN Courses COU ON COU.number = ATC.course_number AND COU.section = ATC.course_section
WHERE APP.level = 7
SELECT *
FROM Applicants APP
JOIN ApplicantsToCourses ATC ON ATC.student_id = APP.student_id
JOIN Courses COU ON COU.number = ATC.course_number AND COU.section = ATC.course_section
WHERE ATC.returning = true