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 emplo