How do the isolation levels REPEATABLE READ and SERIALIZABLE differ in databases?

Transaction isolation levels REPEATABLE READ and SERIALIZABLE differ in the degree of prevention of anomalies during concurrent data access:

• REPEATABLE READ guarantees that if a transaction has read data, subsequent reads of the same data within that transaction will return the same values. This prevents non-repeatable reads but allows phantom reads, where new rows matching the query condition can appear.

• SERIALIZABLE is the strictest isolation level. It ensures complete transaction execution sequence as if transactions were executed one after another. This prevents phantom reads and other anomalies but may reduce performance due to locking and waiting.

Thus, SERIALIZABLE provides stricter isolation and data integrity but potentially at the cost of reduced parallelism compared to REPEATABLE READ.