Alter Tables

1. Make email NOT NULL in customers

ALTER TABLE customers
ALTER COLUMN email SET NOT NULL;

Ensures all future records must have an email.

2. Make username unique in users

ALTER TABLE users ADD CONSTRAINT unique_username UNIQUE (username);

Prevents duplicate usernames.

3. Add CHECK on price in products

ALTER TABLE products ADD CONSTRAINT check_price CHECK (price > 0);

Ensures price is always greater than 0.

4. Set default status in orders

ALTER TABLE orders
ALTER COLUMN status SET DEFAULT 'pending';

Automatically assigns pending if no value is given.

5. Add salary column with constraints

ALTER TABLE employees ADD COLUMN salary INT NOT NULL CHECK (salary > 10000);

Ensures salary is mandatory and above 10,000.

6. Add ON DELETE CASCADE to foreign key

ALTER TABLE employees DROP CONSTRAINT employees_department_id_fkey;

ALTER TABLE employees ADD CONSTRAINT employees_department_id_fkey FOREIGN KEY (department_id) REFERENCES departments(id) ON DELETE CASCADE;

Deletes employees automatically when a department is removed.

7. Remove CHECK constraint on balance

ALTER TABLE accounts DROP CONSTRAINT accounts_balance_check;

Removes restriction on balance.

8. Add composite UNIQUE in payments

ALTER TABLE payments ADD CONSTRAINT unique_user_transaction UNIQUE (user_id, transaction_id);

Ensures no duplicate transaction per user.