What does the Repeatable Read isolation level mean and what problem does it prevent in database transactions?

The Repeatable Read isolation level guarantees that within a single transaction, all re-read data will return the same results.

This prevents the non-repeatable read problem, where data could change between two reads by another transaction.

For example, if transaction A reads a row, then transaction B modifies and commits it, when transaction A repeats the read at Repeatable Read level, it will see the original value, not the changed one.

However, Repeatable Read does not always prevent phantom reads (new rows appearing), for which Serializable level is needed.

Thus, Repeatable Read ensures data stability during repeated reads in a transaction, increasing integrity and predictability.