Can you list the main types of transactional schemes used in relational databases?

In relational databases, the main types of transactional schemes are related to transaction isolation levels, which determine how and when changes made by one transaction become visible to others. The primary isolation levels standardized by SQL include:

• Read Uncommitted — the lowest level of isolation, where a transaction can see uncommitted changes made by other transactions (dirty read).
• Read Committed — a transaction only sees changes that have been committed, preventing dirty reads.
• Repeatable Read — guarantees that repeated reads of the same data within a transaction will yield the same result, preventing non-repeatable reads.
• Serializable — the strictest level of isolation, where transactions are executed as if they were sequential, preventing phantom reads.

There are also transaction management schemes such as two-phase commit (2PC) for distributed transactions, ensuring data consistency across multiple sources.

Example of using isolation levels in SQL:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
-- data operations
COMMIT;