What undesirable effects can occur during concurrent execution of transactional operations?

The following undesirable effects (isolation problems) may occur during concurrent transactional operations:

• Dirty Read: a transaction reads data that has been modified by another transaction but not yet committed. If that transaction rolls back, the data will be incorrect.

• Non-repeatable Read: a transaction reads the same data twice, but another transaction has modified that data between the reads.

• Phantom Read: a transaction executes a query with a condition, and between the queries, another transaction adds or deletes rows that satisfy the condition.

• Lost Update: two transactions read and modify the same data simultaneously, and one transaction's changes overwrite the other's.

These effects are related to the transaction isolation level and can lead to data inconsistency, so it is important to choose the appropriate isolation level and use locking mechanisms.