Drill 19: Improve it — Transactions and locking

CREATE TABLE accounts (
  id SERIAL PRIMARY KEY,
  balance NUMERIC(12, 2) NOT NULL DEFAULT 0
);

INSERT INTO accounts (balance) VALUES (1000.00), (500.00);

BEGIN;

UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;

COMMIT;

BEGIN;

UPDATE accounts SET balance = balance - 9999 WHERE id = 1;

ROLLBACK;