1. Tư Duy Mô Hình Hóa Dữ Liệu & Thiết Kế Schema Chuẩn 3NF (Third Normal Form)

-- THIẾT KẾ SCHEMA QUẢN LÝ BÁN HÀNG CHUẨN 3NF
CREATE TABLE customers (
    id SERIAL PRIMARY KEY,
    full_name VARCHAR(100) NOT NULL,
    email VARCHAR(150) UNIQUE NOT NULL,
    phone VARCHAR(20),
    created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
);

CREATE TABLE orders (
    id SERIAL PRIMARY KEY,
    customer_id INT NOT NULL REFERENCES customers(id) ON DELETE RESTRICT,
    order_date TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
    status VARCHAR(30) DEFAULT 'pending' CHECK (status IN ('pending', 'paid', 'shipped', 'cancelled')),
    total_amount NUMERIC(15, 2) NOT NULL DEFAULT 0.00
);

CREATE TABLE order_items (
    id SERIAL PRIMARY KEY,
    order_id INT NOT NULL REFERENCES orders(id) ON DELETE CASCADE,
    product_name VARCHAR(200) NOT NULL,
    quantity INT NOT NULL CHECK (quantity > 0),
    unit_price NUMERIC(12, 2) NOT NULL
);