BEGIN;

UPDATE accounts SET balance = balance - 100 WHERE id = 1;

UPDATE accounts SET balance = balance + 100 WHERE id = 2;

COMMIT;

BEGIN;

UPDATE employees SET salary = salary * 1.1;

-- Oops, wrong! Undo changes

ROLLBACK;

BEGIN;

DELETE FROM old_records WHERE created_at < '2020-01-01';

-- Check affected rows

SELECT COUNT(*) FROM old_records;

-- If correct: COMMIT, if wrong: ROLLBACK

COMMIT;

BEGIN;

INSERT INTO orders (customer_id, total) VALUES (123, 100);

INSERT INTO order_items (order_id, product_id) VALUES (LAST_INSERT_ID(), 456);

-- If any error occurs, ROLLBACK

COMMIT;

BEGIN: Start transaction

COMMIT: Save changes permanently

ROLLBACK: Undo all changes since BEGIN

Transactions are atomic (all or nothing)

Auto-commit: Each statement is a transaction by default

Explicit transactions override auto-commit

Core references in this topic include WHERE, =, <, >, <=, >=. Learn what each one does, when to use it, and the execution or engine rules that matter.

WHERE

Filters rows before projection and sorting. It decides which rows continue through the query pipeline.

SELECT ... FROM table WHERE condition;

Most performance issues start with a weak WHERE clause or a missing supporting index.

=

Returns rows where the left and right values are exactly equal.

column = value

Use with exact matches. Do not use = NULL.

<, >, <=, >=

Range comparison operators for less-than, greater-than, and inclusive boundary checks.

salary >= 80000

ANY / ALL

Compares one value against every or at least one value from a subquery result.

salary > ALL (SELECT salary FROM interns)