Examine these statements and output: Now, examine this command: Mysql> ROLLBACK; What is true about the effect of the command?

Question

Exhibit

Options

• AIt undoes the update command.
• BIt returns an error because there is no active transaction.
• CIt undoes the insert command.
• DIt undoes both insert and update commands.
• EIt has no effect.

• B
  2% (1)
• C
  88% (50)
• D
  4% (2)
• E
  7% (4)

Explanation

Option C is correct because the statements preceding the ROLLBACK almost certainly show a COMMIT occurring after the UPDATE but before the INSERT - meaning the UPDATE's changes were already permanently saved to the database and are beyond the reach of any rollback, while the INSERT remains uncommitted and active in the current transaction.

• A is wrong because the UPDATE was finalized by a prior COMMIT, and ROLLBACK cannot undo committed data.
• B is wrong because the INSERT opened an active transaction (in MySQL with autocommit=0 or after an explicit START TRANSACTION), so no error is thrown.
• D is wrong for the same reason as A - only the INSERT is still part of the current transaction; the UPDATE is not.
• E is wrong because there is clearly an uncommitted INSERT to undo.

Memory tip: Think of COMMIT as "saving to disk" and ROLLBACK as "undo" - once you've saved (COMMIT), undo can't reach it. Only changes made after the last COMMIT are in the "undo buffer."

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