Inserting, Updating and Deleting

Reading data is safe. Changing it is not. Every write statement deserves a test query first.

INSERT

List the columns explicitly so the statement keeps working if the table gains a column.

INSERT INTO students (name, level, email)
VALUES ('Amina Bello', 'SS2', 'amina@example.com');

INSERT INTO students (name, level)
VALUES
    ('Bola Ade', 'SS1'),
    ('Chidi Okeke', 'SS3');

UPDATE

Always include a WHERE clause. Run the same condition as a SELECT first to see which rows you are about to change.

-- Check first
SELECT id, name, score FROM students WHERE level = 'SS1';

-- Then update
UPDATE students
SET score = score + 5
WHERE level = 'SS1';

DELETE

Delete with a precise condition. To remove every row on purpose, use TRUNCATE, which is faster and resets auto-increment.

DELETE FROM students
WHERE id = 12;

Practice

1. Insert three new courses in one statement.
2. Increase the price of one course by 10%.
3. Delete students who have no email and are inactive.

Hands-on exercise

Write two statements:
1. A SELECT that shows courses priced below 5000.
2. An UPDATE that raises those prices by 10%.
The UPDATE must not touch any other course.

Recap

INSERT adds rows, UPDATE changes them and DELETE removes them. Always preview with SELECT and keep the WHERE clause precise.