How do transactions ensure data consistency?

Transactions ensure data consistency because each one:

1. Checks the database's rules and constraints, foreign keys, unique values, CHECK constraints, triggers. If even one condition is broken, the transaction never commits.
2. Preserves logical integrity, the data stays in a state consistent with business logic after the transaction runs.
3. Uses rollbacks (ROLLBACK), if something fails, the database returns to its last consistent state.

Example: if a transaction is supposed to save an order and deduct an item from stock, but the deduction fails, the whole process gets cancelled. So the database never ends up in a state where the order exists but the stock doesn't.