Tell us about transaction isolation levels and the anomalies they address.

Transaction isolation levels determine how much a transaction can see the changes made by other concurrent transactions. The main isolation levels in the SQL standard:

1. Read Uncommitted — the lowest level. A transaction can see uncommitted changes made by others (dirty read). Anomalies:

   • Dirty Read
   • Non-repeatable Read
   • Phantom Read

2. Read Committed — a transaction only sees committed changes. Dirty reads are prevented, but the following are possible:

   • Non-repeatable Read
   • Phantom Read

3. Repeatable Read — guarantees that repeated reads of the same data within a transaction will yield the same result. It prevents dirty and non-repeatable reads, but phantom reads can occur.

4. Serializable — the strictest level, transactions are executed as if they were sequential. It prevents all anomalies, including phantom reads.

Examples of anomalies:

• Dirty Read: Transaction A reads data modified by transaction B, which has not yet committed and may be rolled back.
• Non-repeatable Read: Transaction A reads data, then transaction B modifies and commits it, and upon re-reading, transaction A sees different values.
• Phantom Read: Transaction A executes a query with a condition, then transaction B adds a new row matching the condition, and upon re-executing the query, transaction A sees the new row.

In Java, you can control the isolation level using JDBC methods, for example:

connection.setTransactionIsolation(Connection.TRANSACTION_REPEATABLE_READ);

Choosing an isolation level involves a trade-off between performance and data consistency.