What types of transaction isolation are there and in which can dirty reads occur?

Types of transaction isolation levels in databases (for example, in SQL standard):

1. Read Uncommitted — the lowest isolation level. Allows dirty reads, where a transaction reads data that has been modified but not yet committed by another transaction.
2. Read Committed — prevents dirty reads but allows non-repeatable reads and phantom reads.
3. Repeatable Read — prevents dirty reads and non-repeatable reads but allows phantom reads.
4. Serializable — the highest isolation level, prevents all the above anomalies, including phantom reads.

Dirty reads are only possible at the Read Uncommitted level.

Example of a dirty read:

• Transaction A modifies data but does not commit.
• Transaction B reads this modified data.
• If Transaction A rolls back, the data read by Transaction B becomes incorrect.

Most DBMSs either do not support the Read Uncommitted level or do not use it by default due to the risk of errors.