Rel.d

⏺ Here's a Tutorial D/Rel design for tracking book loans with a max 5 books per client constraint:

// Base relvars
VAR Client REAL RELATION {client_id INT, name CHAR} KEY {client_id};
VAR Book REAL RELATION {book_id INT, title CHAR} KEY {book_id};
VAR Loan REAL RELATION {client_id INT, book_id INT, loan_date DATE} 
  KEY {book_id} // A book can only be loaned to one client at a time
;

// Foreign key constraints
CONSTRAINT Loan_Client_FK
  Loan {client_id} ⊆ Client {client_id};

CONSTRAINT Loan_Book_FK
  Loan {book_id} ⊆ Book {book_id};

// Max 5 books per client constraint
CONSTRAINT Max5BooksPerClient
  IS_EMPTY(
     (SUMMARIZE Loan BY {client_id} ADD (COUNT() AS loan_count))
     WHERE loan_count > 5
   );

solution.sql

CREATE TABLE clients (
    id SERIAL PRIMARY KEY,
    name VARCHAR(255) NOT NULL
);

CREATE TABLE books (
    id SERIAL PRIMARY KEY,
    title VARCHAR(255) NOT NULL
);

CREATE TABLE borrowings (
    id SERIAL PRIMARY KEY,
    client_id INT NOT NULL REFERENCES clients(id),
    book_id INT NOT NULL REFERENCES books(id),
    borrowed_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    returned_at TIMESTAMP,
    UNIQUE(book_id, returned_at)  -- prevent double-borrowing active books
);

CREATE OR REPLACE FUNCTION active_borrow_count(p_client_id INT)
RETURNS INT AS $$
    SELECT COUNT(*)::INT FROM borrowings
    WHERE client_id = p_client_id AND returned_at IS NULL;
$$ LANGUAGE sql STABLE;

ALTER TABLE borrowings
    ADD CONSTRAINT max_5_books
    CHECK (active_borrow_count(client_id) < 5);