SQL: Incorrect by Construction(chreke.com)

  BEGIN TRANSACTION;

  IF EXISTS (
    SELECT 1
    FROM account_balances WITH (UPDLOCK, HOLDLOCK)
    WHERE owner = 'alice'
      AND balance >= 10
  )
  BEGIN
    INSERT INTO account_ledger (owner, amount, memo)
    VALUES
        ('alice', -10, 'transfer to bob'),
        ('bob',    10, 'transfer from alice');
  END

  COMMIT TRANSACTION;

  BEGIN TRANSACTION;

  UPDATE accounts WITH (UPDLOCK)
  SET balance = balance - 10
  WHERE owner = 'alice'
  AND balance >= 10;

  IF @@ROWCOUNT = 1
  BEGIN
    UPDATE accounts
    SET balance = balance + 10
    WHERE owner = 'bob';
  END

  COMMIT TRANSACTION;

  UPDATE accounts  
  SET balance = balance - 10  
  WHERE owner = 'alice' AND balance >= 10;

  INSERT INTO foo(num) (
    SELECT 1 WHERE NOT EXISTS (
      SELECT * FROM foo WHERE num = 1
    )
  );

  set @balance = (....