Modifying Tables in SQL using ALTER

1. Making a Column NOT NULL Suppose we already created a customers table, but now we want to ensure that email is always provided. ALTER TABLE customers ALTER COLUMN email SET NOT NULL; 2. Adding UNIQUE Constraint To make sure no two users have the same username: ALTER TABLE users ADD CONSTRAINT unique_username UNIQUE (username); 3. Adding CHECK Constraint We want to ensure product price is always greater than 0: ALTER TABLE products ADD CONSTRAINT check_price CHECK (price > 0); 4. Setting DEFAULT Value To make status = 'pending' by default: ALTER TABLE orders ALTER COLUMN status SET DEFAULT 'pending'; 5. Adding a Column with Constraints Now we add a salary column to employees: ALTER TABLE employees ADD COLUMN salary DECIMAL(10,2) NOT NULL CHECK (salary > 10000); 6. Modifying Foreign Key with CASCADE To automatically delete employees when a department is deleted: ALTER TABLE employees DROP CONSTRAINT employees_department_id_fkey; ALTER TABLE employees ADD CONSTRAINT employees_dep