Transaction Control Language (TCL) Flashcards

Which ACID property ensures concurrent transactions do not interfere with each other?

Answer: Isolation

Isolation keeps concurrent transactions from seeing each other's intermediate states.

A non-repeatable read occurs when:

Answer: A re-read row returns different data due to another committed update

Non-repeatable reads happen when another transaction modifies and commits a row between two reads.

Which isolation level is the default in many systems like PostgreSQL and Oracle?

Answer: READ COMMITTED

READ COMMITTED is the default isolation level in PostgreSQL and Oracle.

Can a SELECT statement be rolled back by ROLLBACK?

Answer: No, SELECT does not change data

SELECT reads data and makes no changes, so there is nothing for ROLLBACK to undo.

After ROLLBACK is issued on a transaction with no savepoints, the transaction is:

Answer: Fully undone and ended

A plain ROLLBACK undoes all changes and ends the transaction.

Which command would you use to undo only the last few operations while keeping earlier ones in a transaction?

Answer: ROLLBACK TO SAVEPOINT

ROLLBACK TO SAVEPOINT undoes changes after a named savepoint while preserving earlier work.

Why is grouping related DML statements into a single transaction important?

Answer: To ensure all-or-nothing data integrity

Wrapping related changes in one transaction guarantees they all succeed or all fail together.