Which of the following is an incorrect statement?

Question

BlackTom AI · Accepted Answer

First, let’s restate the question and options to be crystal clear about what’s under consideration.
Question: Which of the following is an incorrect statement?
Options:
A) A commit is issued implicitly when a user properly logs out of the system.
B) A commit is issued implicitly when a DDL command is executed.
C) A commit is issued automatically when a DML command is executed
D) Already executed COMMIT can be resversed by using Transaction Control Language.

Now, let’s analyze each option in turn.

Option A: 'A commit is issued implicitly when a user properly logs out of the system.' In many database environments, logging out does not by itself perform a commit; it depends on the client/session configuration. If autocommit is off (which is common in many setups), closing a session could roll back uncommitted work rather than commit it. Therefore this statement is not universally true and is at best misleading. It is not a reliable rule across systems, so this statement is incorrect in the general sense.

Option B: 'A commit is issued implicitly when a DDL command is executed.' In several database systems, particularly Oracle and some others, DDL statements (like CREATE, ALTER, DROP) cause an implicit commit to occur because they change the schema and require a consistent state. This behavior is widely taught and documented, so this statement is generally considered correct.

Option C: 'A commit is issued automatically when a DML command is executed.' The truth of this depends on the environment and configuration. In many databases, DML statements (INSERT, UPDATE, DELETE) do not automatically commit; they require an explicit COMMIT or a transactional setting like autocommit enabled. In systems where autocommit is not on by default, this statement is false. Therefore, as a universal rule, this statement is not reliable and is incorrect in general.

Option D: 'Already executed COMMIT can be resversed by using Transaction Control Language.' Once a COMMIT has been executed, the changes are finalized in the current transaction. In standard SQL and most DBMS implementations, you cannot reverse a committed transaction using normal TCL commands like ROLLBACK, unless you have a mechanism like point-in-time recovery, flashback features, or a separate undo log. In typical practice, this statement is incorrect because you cannot simply 'reverse' a committed transaction with standard TCL after the fact.

Putting the pieces together, several statements are not universally correct:
- A is not reliably true because logout does not guarantee a commit in all environments.
- C is not universally true because DML often does not auto-commit unless autocommit is enabled.
- D is not true in the normal sense since a committed transaction cannot be rolled back with ordinary TCL once committed.

Option B stands out as the commonly accepted correct statement about implicit commits tied to DDL, while A, C, and D each rest on conditions or interpretations that aren’t universally valid across DBMS implementations. The key misconception in D is that a committed state can always be reversed with TCL; in practice, once committed, reversal generally requires more advanced recovery mechanisms rather than a simple rollback.

In summary, each option has caveats, but B reflects a standard, widely recognized behavior, whereas A, C, and D are incorrect in typical transactional semantics across mainstream database systems.

Which of the following is an incorrect statement?单项选择题

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