== Annual analysis of course enrollments, completions and certificates
{{{
SELECT
    EXTRACT(YEAR FROM e.enroll_date) AS Year,
    COUNT(DISTINCT e.user_id) AS Total_Students,
    COUNT(DISTINCT e.enrollment_id) AS Total_Enrollments,
    COUNT(DISTINCT CASE WHEN e.completion_status = 'COMPLETED' THEN e.enrollment_id END) AS Completed_Enrollments,
    COUNT(DISTINCT cert.certificate_id) AS Total_Certificates,
    ROUND(
        COUNT(DISTINCT CASE WHEN e.completion_status = 'COMPLETED' THEN e.enrollment_id END)::DECIMAL
        / NULLIF(COUNT(DISTINCT e.enrollment_id), 0) * 100, 2) AS Completion_Rate_Percentage,
    ROUND(AVG(e.progress_percentage), 2) AS Avg_Progress_Percentage
FROM enrollment e
LEFT JOIN certificate cert
    ON cert.enrollment_id = e.enrollment_id
GROUP BY EXTRACT(YEAR FROM e.enroll_date)
ORDER BY Year;
}}}

==== Solution Relational Algebra

- E(enrollment_id, user_id, course_id, enroll_date, completion_status, progress_percentage)
- CERT(certificate_id, enrollment_id, issue_date, certificate_code, status)


**JOIN on all tables**

J1 ← E ⟕ E.enrollment_id = CERT.enrollment_id CERT

**Projection with date transformation**

E' ← π enrollment_id, user_id, completion_status, progress_percentage, certificate_id, YEAR(enroll_date) → Year (J1)

**Grouping and aggregate calculations**

G ← Year γ
COUNT(DISTINCT user_id) → Total_Students,
COUNT(DISTINCT enrollment_id) → Total_Enrollments,
COUNT(DISTINCT [completion_status = 'COMPLETED'] enrollment_id) → Completed_Enrollments,
COUNT(DISTINCT certificate_id) → Total_Certificates,
AVG(progress_percentage) → Avg_Progress_Percentage
(E')

**Final projection and percentage calculation**

P ← π Year, Total_Students, Total_Enrollments, Completed_Enrollments, Total_Certificates,
(Completed_Enrollments / Total_Enrollments) * 100 → Completion_Rate_Percentage,
Avg_Progress_Percentage (G)

**Chronological ordering by year**
R_final ← τ Year (P)


== Instructor performance analysis by number of courses, students, modules and lessons 

{{{
SELECT
    i.id AS Instructor_ID,
    ue.first_name || ' ' || ue.last_name AS Instructor_Name,
    COUNT(DISTINCT c.course_id) AS Total_Courses,
    COUNT(DISTINCT e.user_id) AS Total_Students,
    COUNT(DISTINCT m.module_id) AS Total_Modules,
    COUNT(DISTINCT l.lesson_id) AS Total_Lessons,
    ROUND(AVG(c.price), 2) AS Avg_Course_Price
FROM instructors i
JOIN user_entity ue
    ON ue.id = i.id
JOIN course c
    ON c.instructor_id = i.id
LEFT JOIN enrollment e
    ON e.course_id = c.course_id
LEFT JOIN module m
    ON m.course_id = c.course_id
LEFT JOIN lesson l
    ON l.module_id = m.module_id
GROUP BY
    i.id,
    ue.first_name,
    ue.last_name
ORDER BY
    Total_Students DESC,
    Total_Courses DESC;
}}}

==== Solution Relational Algebra

- I(id)
- U(id, first_name, last_name, ...)
- C(course_id, name, price, status, instructor_id)
- E(enrollment_id, user_id, course_id, enroll_date, completion_status, progress_percentage)
- M(module_id, course_id, title, description)
- L(lesson_id, module_id, title, material)

**JOIN on all tables:**

J1 ← I ⋈ I.id = U.id U

J2 ← J1 ⋈ I.id = C.instructor_id C

J3 ← J2 ⋈ C.course_id = E.course_id E

J4 ← J3 ⋈ C.course_id = M.course_id M

J5 ← J4 ⋈ M.module_id = L.module_id L

**Projection with instructor full name**

F1 ← π id, CONCAT(first_name, ' ', last_name) → Instructor_Name, course_id, user_id, module_id, lesson_id, price (J5)

**Grouping and aggregate calculations**

G ← id, Instructor_Name γ COUNT(DISTINCT course_id) → Total_Courses, COUNT(DISTINCT user_id) → 
Total_Students, COUNT(DISTINCT module_id) → Total_Modules, COUNT(DISTINCT lesson_id) → 
Total_Lessons, AVG(price) → Avg_Course_Price (F1)

**Ordering by number of students and courses**

R_final ← τ Total_Students DESC, Total_Courses DESC (G)


== Annual subscription and revenue analysis by subscription plan

{{{
SELECT
    EXTRACT(YEAR FROM us.start_date) AS Year,
    sp.name AS Plan_Name,
    COUNT(DISTINCT us.subscription_id) AS Total_Subscriptions,
    COUNT(DISTINCT us.user_id) AS Total_Users,
    COUNT(DISTINCT p.payment_id) AS Total_Payments,
    ROUND(SUM(COALESCE(p.amount, 0)), 2) AS Total_Revenue,
    ROUND(AVG(COALESCE(p.amount, 0)), 2) AS Avg_Payment_Amount
FROM user_subscription us
JOIN subscription_plan sp
    ON sp.plan_id = us.plan_id
LEFT JOIN payment p
    ON p.subscription_id = us.subscription_id
GROUP BY
    EXTRACT(YEAR FROM us.start_date),
    sp.name
ORDER BY
    Year,
    Total_Revenue DESC;
}}}

==== Solution Relational Algebra

- US(subscription_id, user_id, plan_id, start_date, end_date, status)
- SP(plan_id, name, price, duration_months, description, access_type)
- P(payment_id, user_id, subscription_id, amount)


**JOIN on all tables**

J1 ← US ⋈ US.plan_id = SP.plan_id SP

J2 ← J1 ⋈ US.subscription_id = P.subscription_id P

**Projection with date transformation**

F1 ← π subscription_id, user_id, payment_id, amount, name, YEAR(start_date) → Year (J2)

**Renaming attribute**

F2 ← ρ Plan_Name/name (F1)

**Grouping and aggregate calculations**

G ← Year, Plan_Name γ COUNT(DISTINCT subscription_id) → Total_Subscriptions, COUNT(DISTINCT user_id)
→ Total_Users, COUNT(DISTINCT payment_id) → Total_Payments, SUM(amount) → Total_Revenue, 
AVG(amount) → Avg_Payment_Amount (F2)

**Chronological ordering and descending revenue ordering**

R_final ← τ Year, Total_Revenue DESC (G)


== Course quiz success analysis by attempts, average score and pass rate

{{{
SELECT
    c.course_id,
    c.name AS Course_Name,
    COUNT(DISTINCT qa.attempt_id) AS Total_Attempts,
    COUNT(DISTINCT qa.user_id) AS Total_Students_Attempted,
    ROUND(AVG(qa.score), 2) AS Avg_Score,
    COUNT(DISTINCT CASE WHEN qa.score >= q.passing_score THEN qa.attempt_id END) AS Passed_Attempts,
    ROUND(
        COUNT(DISTINCT CASE WHEN qa.score >= q.passing_score THEN qa.attempt_id END)::DECIMAL
        / NULLIF(COUNT(DISTINCT qa.attempt_id), 0) * 100,
        2
    ) AS Pass_Rate_Percentage
FROM course c
JOIN module m
    ON m.course_id = c.course_id
JOIN lesson l
    ON l.module_id = m.module_id
JOIN quiz q
    ON q.lesson_id = l.lesson_id
LEFT JOIN quiz_attempt qa
    ON qa.quiz_id = q.quiz_id
GROUP BY
    c.course_id,
    c.name
ORDER BY
    Pass_Rate_Percentage DESC,
    Avg_Score DESC;

}}}


==== Solution Relational Algebra

- C(course_id, name, price, status, instructor_id)
- M(module_id, course_id, title, description)
- L(lesson_id, module_id, title, material)
- Q(quiz_id, total_points, passing_score, lesson_id)
- QA(attempt_id, score, attempt_date, user_id, quiz_id)

**JOIN on all tables:**

J1 ← C ⋈ C.course_id = M.course_id M

J2 ← J1 ⋈ M.module_id = L.module_id L

J3 ← J2 ⋈ L.lesson_id = Q.lesson_id Q

J4 ← J3 ⋈ Q.quiz_id = QA.quiz_id QA

**Projection and renaming**

F1 ← π course_id, name, attempt_id, user_id, score, passing_score (J4)

F2 ← ρ Course_Name/name (F1)

**Grouping and aggregate calculations**

G ← course_id, Course_Name γ COUNT(DISTINCT attempt_id) → Total_Attempts, COUNT(DISTINCT user_id) 
→ Total_Students_Attempted, AVG(score) → Avg_Score, COUNT(DISTINCT [score >= passing_score] 
attempt_id) → Passed_Attempts (F2)

**Final projection and percentage calculation**

P ← π course_id, Course_Name, Total_Attempts, Total_Students_Attempted, Avg_Score, Passed_Attempts, 
(Passed_Attempts / Total_Attempts) * 100 → Pass_Rate_Percentage (G)

**Ordering by pass rate and average score**

R_final ← τ Pass_Rate_Percentage DESC, Avg_Score DESC (P)