Can you explain the principle of the Repeatable Read transaction isolation level in databases?

The Repeatable Read transaction isolation level guarantees that within a single transaction, all repeated reads of the same data will return the same results. This means that if a transaction has read a row from the database, it will not see changes made by other transactions when reading the same row again within the same transaction.

Main features:

• Prevents non-repeatable reads.
• May allow phantom reads (new rows added by other transactions can appear in subsequent queries).

Example: if a transaction reads an account balance, the balance will not change upon subsequent reads within the same transaction, even if another transaction updates it after the first read.

This is achieved through locking or data versioning, depending on the DBMS.