What physical types of connections (joints) exist at the physical level?

In the context of database systems, there are usually four main levels of transaction isolation defined by the SQL standard:

1. Read Uncommitted: The lowest level. A transaction may see uncommitted changes made by other transactions (dirty reads).

2. Read Committed: A transaction only sees committed data. This prevents dirty reads but allows non-repeatable reads.

3. Repeatable Read: Ensures that if a transaction reads data, subsequent reads will see the same data, preventing non-repeatable reads. However, phantom reads can still occur.

4. Serializable: The strictest level, where transactions are executed in a way that results are as if they were run sequentially. It prevents all anomalies, including phantom reads.

Each level balances between performance and data integrity.