Oracle SQL Oracle SQL Transactions and Concurrency Control 3

Question 1

What is the default transaction isolation level in Oracle Database?

Accepted Answer

READ COMMITTED

Answer

Oracle's default isolation level is READ COMMITTED, where each query sees only data that was committed before the query began.

Question 2

Which read phenomenon is prevented by Oracle's READ COMMITTED isolation level?

Accepted Answer

Dirty reads

Answer

READ COMMITTED prevents dirty reads because queries only see data that was committed before the query started, never uncommitted data from other transactions.

Question 3

What does a dirty read mean in the context of database transactions?

Accepted Answer

Reading uncommitted data from another transaction that may later be rolled back

Answer

A dirty read occurs when a transaction reads data modified but not yet committed by another transaction, which may be rolled back, making the read data invalid.

Question 4

Which Oracle SQL statement correctly sets the current transaction's isolation level to SERIALIZABLE?

Accepted Answer

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE

Answer

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE must be the first statement in a transaction and sets the isolation level for that transaction only.

Question 5

What is a phantom read in the context of database transactions?

Accepted Answer

A transaction re-executes a range query and finds new rows inserted and committed by another transaction since the first execution

Answer

A phantom read occurs when a transaction re-executes a range query and finds new rows ('phantoms') inserted and committed by another transaction since the first execution.

Oracle SQL Practice Test

Oracle SQL Practice Test

Oracle SQL Oracle SQL Transactions and Concurrency Control 3