What levels of transaction isolation exist in SQL?

Transaction isolation levels in SQL determine the degree of protection of data from concurrent transactions. They affect potential issues: dirty read, non-repeatable read, and phantom read.

The SQL standard defines four levels of isolation:

1. Read Uncommitted: The lowest level. A transaction can read uncommitted (dirty) data from other transactions.

   • Allows: dirty read, non-repeatable read, phantom read.

2. Read Committed: A transaction can only read committed data. Dirty reads are not possible.

   • Allows: non-repeatable read, phantom read.

3. Repeatable Read: The transaction is guaranteed to see the same data upon repeated reads. Data read within a transaction cannot be changed or deleted by another transaction until it completes.

   • Allows: phantom read.

4. Serializable: The highest level. Transactions are executed sequentially as if they were run one after another. It fully prevents all the above issues.

   • Allows: none of the listed problems.

Summary table of problems and isolation levels:

Problem	Read Uncommitted	Read Committed	Repeatable Read	Serializable
Dirty read	Possible	Not possible	Not possible	Not possible
Non-repeatable read	Possible	Possible	Not possible	Not possible
Phantom read	Possible	Possible	Possible	Not possible

Choosing an isolation level depends on the application's requirements for data consistency and performance. Higher levels provide greater integrity but may reduce performance due to increased locking. The default level often depends on the specific DBMS (for example, PostgreSQL and Oracle default to Read Committed, MySQL defaults to Repeatable Read).