I believe the answer is wrong for Exercise 3 on page 5 of SQLcourse2 dealing with the HAVING clause. The question asks to,

"Select the customerid, number of orders they made, and the sum of their orders if they purchased more than 1 item."
It doesn't ask if they made more than one purchase. Therefore, I believe the answer should be like this:

SELECT customerid, COUNT(customerid), SUM(price)
FROM items_ordered
GROUP BY customerid
HAVING SUM(quantity) > 1;

Instead of counting the customerid, you should sum the quantity, which does include a person who ordered only once, but bought more than one item.