Today I practiced ALTER TABLE in SQL.
This is used when we want to change an existing table.
1. Make Email NOT NULL (customers)
ALTER TABLE customers
MODIFY email VARCHAR(100) NOT NULL;
Now email is required
2. Make Username UNIQUE (users)
ALTER TABLE users
ADD CONSTRAINT unique_username UNIQUE (username);
No duplicate usernames
3. Add CHECK on Price (products)
ALTER TABLE products
ADD CONSTRAINT check_price CHECK (price > 0);
price must be > 0
4. Default Status (orders)
ALTER TABLE orders
ALTER COLUMN status SET DEFAULT 'pending';
if not given → pending
5. Add Salary Column (employees)
ALTER TABLE employees
ADD salary INT NOT NULL CHECK (salary > 10000);
salary required + > 10000
6. Auto Delete Employees (FK)
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;
delete department → employees also deleted
7. Remove CHECK (accounts)
ALTER TABLE accounts
DROP CONSTRAINT check_balance;
removed balance rule
8. Unique Combination (payments)
ALTER TABLE payments
ADD CONSTRAINT unique_payment UNIQUE (user_id, transaction_id);
same pair not allowed twice
Final Thought
ALTER TABLE = update your table rules
Very useful in real projects
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