What are the differences in the transaction isolation levels in MySQL?

There are four transaction isolation levels in MySQL: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, and SERIALIZABLE. The differences between these levels mainly lie in the restrictions and rules for reading and writing data during concurrent access.

1. READ UNCOMMITTED: The lowest level of isolation that allows transactions to read data that has not been committed yet. This may result in reading dirty data and inconsistent information if other transactions are modifying the data.
2. READ COMMITTED: stricter than READ UNCOMMITTED, ensures that transactions can only read data that has already been committed. If a transaction modifies and commits data that is then re-read by another transaction, the latter will receive the most up-to-date value.
3. REPEATABLE READ is the default isolation level in MySQL. It ensures that within the same transaction, multiple reads of the same data will result in consistent results, even if other transactions modify the data. However, because uncommitted data is not visible to other transactions within a transaction, it may lead to phantom reads, where the data set retrieved in one transaction is inconsistent with a previous data set retrieved.
4. SERIALIZABLE: The highest level of isolation, ensures data consistency by forcing transactions to be executed serially. While serial execution can prevent phantom reads, it can also reduce database concurrency performance.