Alter Tables

1. Make email NOT NULL in customers
   ALTER TABLE customers
   ALTER COLUMN email SET NOT NULL;

2. Make username UNIQUE in users
   ALTER TABLE users
   ADD CONSTRAINT unique_username UNIQUE (username);

3. Add CHECK (price > 0) in products
   ALTER TABLE products
   ADD CONSTRAINT price_check CHECK (price > 0);

4. Set default 'pending' in orders
   ALTER TABLE orders
   ALTER COLUMN status SET DEFAULT 'pending';

5. Add salary column in employees
   ALTER TABLE employees
   ADD COLUMN salary INT NOT NULL CHECK (salary > 10000);

6. Modify foreign key with ON DELETE CASCADE
   First drop old constraint (name may vary)
   ALTER TABLE employees
   DROP CONSTRAINT employees_department_id_fkey;
   Then add new one
   ALTER TABLE employees
   ADD CONSTRAINT fk_department
   FOREIGN KEY (department_id)
   REFERENCES departments(id)
   ON DELETE CASCADE;

7. Remove CHECK constraint from accounts
   First find constraint name, then:
   ALTER TABLE accounts
   DROP CONSTRAINT accounts_balance_check;

8. Add UNIQUE (user_id, transaction_id) in payments
   ALTER TABLE payments
   ADD CONSTRAINT unique_payment UNIQUE (user_id, transaction_id);