What is a transaction, and what are the four characteristics of a transaction? What are the isolation levels?

1) Transaction (Transaction): is the smallest unit of work for a database operation, a series of operations performed as a single logical unit of work, which are submitted to the system as a whole, either executed or not executed; a transaction is a set of operations that cannot be re-partitioned (the Working Logic unit);

2) A transaction has 4 properties, often called an acid attribute:

3) atomicity (atomicity): A transaction is a complete operation. The steps of a transaction are non-divided, either executed or not.

4) Consistency (consistency): When the transaction completes, the data must be in a consistent state.

5) Isolation (isolation): All concurrent transactions that modify data are isolated from each other, indicating that the transaction must be independent and that it should not rely on or affect other transactions in any way.

6) Persistence (durability): After a transaction is completed, its modifications to the database are persisted and the transaction log keeps the transaction permanent.

7) Isolation level is divided into: readuncommitted (read UNCOMMITTED), read Committed (read commit), repeatable read (can be read repeatedly), Serializable (serialization) of these 4 levels