Can you explain the different levels of transaction isolation in SQL and how they differ from each other?

In SQL, there are four main levels of transaction isolation that determine how and when changes made by one transaction become visible to others:

1. Read Uncommitted — allows reading data that has been modified but not yet committed by other transactions. This can lead to "dirty reads".

2. Read Committed — reads only data that has been committed. Prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read — guarantees that data read in a transaction will not change until the transaction ends. Prevents dirty reads and non-repeatable reads.

4. Serializable — the strictest level.

• Transactions are executed as if they were sequential.
• Prevents all types of read anomalies, including phantom reads.

Choosing the right level affects the balance between performance and data consistency. For example, high concurrency systems often use Read Committed for better performance, while critical systems use Serializable.