SELECT
    order_id,
    IF_NULL(shipper_id, ‘Not Assigned’) AS shipper
    FROM orders
    image.png
    SELECT
    order_id,
    COALESCE(shipper_id, comments, ‘Not Assigned’) AS shipper
    FROM orders
    COALESCE函数是指如果shipper_id为空则返回comments里面的内容，
    如果comments也是空的就返回’Not Assigned’

    SELECT
    CONCAT(first_name, ‘ ‘, last_name) AS customer,
    IFNULL(phone, ‘Unknown’) AS phone,
    FROM customers