Create Tables

1. Students table (id unique)
   CREATE TABLE students (
   id SERIAL PRIMARY KEY,
   name TEXT,
   age INT
   );

2. Employees table (name & email not null)
   CREATE TABLE employees (
   id SERIAL PRIMARY KEY,
   name TEXT NOT NULL,
   email TEXT NOT NULL,
   phone_number TEXT
   );

3. Users table (username & email unique)
   CREATE TABLE users (
   id SERIAL PRIMARY KEY,
   username TEXT UNIQUE,
   email TEXT UNIQUE
   );

4. Products table (price > 0, stock ≥ 0)
   CREATE TABLE products (
   id SERIAL PRIMARY KEY,
   name TEXT,
   price DECIMAL CHECK (price > 0),
   stock INT CHECK (stock >= 0)
   );

5. Orders table (default + timestamp)
   CREATE TABLE orders (
   id SERIAL PRIMARY KEY,
   status TEXT DEFAULT 'pending',
   created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
   );

6. Accounts table
   CREATE TABLE accounts (
   id SERIAL PRIMARY KEY,
   account_number TEXT UNIQUE NOT NULL,
   balance INT CHECK (balance >= 0)
   );

7. Enrollments table (unique combination)
   CREATE TABLE enrollments (
   student_id INT,
   course_id INT,
   PRIMARY KEY (student_id, course_id)
   );

8. Departments & Employees (foreign key)
   CREATE TABLE departments (
   id SERIAL PRIMARY KEY,
   name TEXT
   );

CREATE TABLE employees (
id SERIAL PRIMARY KEY,
name TEXT,
department_id INT,
FOREIGN KEY (department_id) REFERENCES departments(id)
);

1. Foreign key with cascade CREATE TABLE departments ( id SERIAL PRIMARY KEY, name TEXT );

CREATE TABLE employees (
id SERIAL PRIMARY KEY,
name TEXT,
department_id INT,
FOREIGN KEY (department_id)
REFERENCES departments(id)
ON DELETE CASCADE
ON UPDATE CASCADE
);