Compare the SQL transaction isolation levels.

SQL defines four isolation levels that trade consistency against concurrency. They are usually described by which anomalies they permit.

• Read uncommitted: dirty reads allowed, sees uncommitted changes.
• Read committed: no dirty reads, but non-repeatable reads and phantoms can occur. The common default in PostgreSQL and Oracle.
• Repeatable read: rows read stay stable within the transaction, but phantoms may appear. MySQL InnoDB's default, implemented with MVCC.
• Serializable: transactions behave as if run one at a time, preventing all three anomalies, at the cost of blocking or aborts.

level              dirty  non-repeatable  phantom
read uncommitted    yes        yes           yes
read committed      no         yes           yes
repeatable read     no         no            yes
serializable        no         no            no

Implementations vary: MVCC snapshots give readers a consistent view without blocking writers, while lock-based systems block. Higher isolation reduces anomalies but increases contention, deadlocks and retries, so choose per transaction rather than globally.