Which of the following statements about COMMIT and ROLLBACK commands is not correct?

Question

BlackTom AI · Accepted Answer

Let’s break down each statement about COMMIT and ROLLBACK and assess its accuracy in common database behavior.
Option 1: "All DML commands (INSERT, UPDATE, DELETE) are implicitly committed and cannot be rolled back." This claim is not universally true. In databases that use explicit transaction control, DML statements are not implicitly committed; they are part of an open transaction that can be rolled back until a COMMIT is issued. The assumption of implicit commit by default depends on the database’s autocommit setting. Therefore, this statement is misleading and not correct in many environments.
Option 2: "All DDL commands (CREATE, TRUNCATE, ALTER TABLE) are implicitly committed and cannot be rolled back." In many database systems, DDL statements cause an implicit commit, and once executed, their effects cannot be rolled back in the same session. This makes the statement generally accurate for those systems, though there can be exceptions depending on the RDBMS and configuration. Overall, it reflects a common behavior that DDL is not easily rolled back.
Option 3: "A ROLLBACK command cannot reverse changes made before the last COMMIT." This is correct in standard transactional semantics: once a COMMIT has been executed, the changes are durable and cannot be undone by a subsequent ROLLBACK. ROLLBACK only reverses uncommitted work since the last COMMIT (or since the start of the transaction). This statement aligns with typical transaction behavior.
Option 4: "all of the above" This would be chosen only if every preceding statement were correct. Since option 1 is not universally correct (and thus makes the compound option inaccurate in many contexts), this option is not a true characterization of the statements.
In summary, option 1 is the problematic one because it asserts universal implicit commitment and non-rollbackability of DML, which contradicts the typical explicit-transaction model used in many RDBMS. The other statements reflect common transactional behavior for DDL and ROLLBACK semantics, though note that implementations can vary by database system and settings.

Which of the following statements about COMMIT and ROLLBACK commands is not correct?单项选择题

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