BEGIN;
do all the work for a single action
COMMIT;

BEGIN;
 if not enough funds, ROLLBACK and exit
 UPDATE to take money from one account
 UPDATE to add that money to another account
 INSERT into some log or audit trail to track all transactions
 COMMIT;

BEGIN;
SELECT some stuff from a row FOR UPDATE;
test the stuff, such as account balance
UPDATE that same row;
COMMIT;