Practise · The error does not undo the first write

Opening your saved work…

The reading

Both or neither

BEGIN;
INSERT INTO lessons VALUES (5, 1, 3, '2026-03-10', 60);
UPDATE guests SET balance = balance - 60 WHERE id = 1;
SELECT changes() AS guests_debited;
COMMIT;

Desktop only. One booking, one debit, one decision.

A coaching booking at Harbor Snow is two writes: insert the booking, debit the guest's prepaid balance. If the second one fails, the first one is a booking nobody paid for. A transaction makes the two succeed or fail together: BEGIN, both writes, check changes() and the balance, then COMMIT when it is right or ROLLBACK when it is not.

The part people get wrong: a failing statement inside a transaction does not undo the ones before it. When the debit is refused by the balance's CHECK, the transaction is still open and the booking insert is still pending. Nothing is committed and nothing is undone until you say which. The error is information; ROLLBACK is the decision.

The browser playground runs one statement at a time on a copy, so it cannot hold a transaction open. Transactions are a desktop skill: open the Harbor Snow kit in the sqlite3 shell, enter each statement separately, and watch the balances between them.

Rule of thumb. Inside a transaction an error pauses; it does not undo. You decide with COMMIT or ROLLBACK.

Going deeper (optional, not on the exam)

The sqlite3 shell's .bail on stops a script at the first error. That is not a rollback: everything before the error that was not inside a transaction has already happened. Use a fresh rehearsal copy to retry.

The error does not undo the first write · BEGIN, COMMIT, ROLLBACK (desktop) · level 5 of 6

The error does not undo the first write

Level 5: Repair it · Exercise 1 of 1

A 70-dollar transfer from guest 2 to guest 3 failed halfway. Guest 3 is 70 richer. Guest 2 is unchanged.

Run this on a fresh copy in the desktop shell, one statement at a time; the second UPDATE is refused by the balance CHECK. Write down both balances and the exact error, then fix the handling: the right next statement is the one that leaves both balances exactly as they were. Do not replace ROLLBACK with COMMIT.

This one runs in desktop SQLite 3.25 or newer, not in the browser. Download the kit

The query that runs and lies

SELECT id, balance FROM guests WHERE id IN (2, 3) ORDER BY id;
BEGIN;
UPDATE guests SET balance = balance + 70 WHERE id = 3;
UPDATE guests SET balance = balance - 70 WHERE id = 2;
-- the second update is refused; the first is pending

Your repair, starting from the query that lies

What the shell returned: paste the result, or say in a sentence what it showed

What would have happened to guest 3's balance without BEGIN?

ReviewBoth or neither, paragraph 2
Opens after the hint or a second check.

I would like help with this exercise

Practice here is evidence for you and your instructor, never a grade. Grades live in Canvas.