Before writing SQL, let's look at what we're building. The system has three main tables:

students                 books
    │                       │
    │ student_id            │ book_id
    │                       │
    └──────────┐   ┌─────────┘
               ▼   ▼
              borrow

CLIFF AMADEUS → Jurassic Park
JAN XAVIER    → Project Hail Mary
RON EDMUND    → 1984

1. One student can have many borrowing records.
2. One book can appear in many borrowing records over time.
3. Each borrowing record belongs to one student and one book.
4. A borrow_return_date of NULL means the book has not been returned yet.
5. A non-NULL borrow_return_date means the book has been returned.

Think of borrow as a transaction table. It records who borrowed which book and when.