Mysql #7

-- Problem #1
SELECT DISTINCT category_name
FROM categories WHERE category_id IN
(SELECT  category_id
FROM products
WHERE products.category_id = categories.category_id )
ORDER BY category_name;

-- Problem #2
SELECT product_name,
	   list_price
FROM products
WHERE list_price >
    (SELECT AVG(list_price)
    FROM products WHERE list_price > 0 )
    ORDER BY list_price ASC;

-- Problem #3
SELECT category_name
FROM categories WHERE NOT EXISTS
(SELECT category_id
FROM products WHERE products.category_id = categories.category_id);

-- Problem #4
SELECT
	email_address,
	order_id,
    SUM(item_price)
FROM customers
JOIN orders
GROUP BY email_address;

-- Problem #5
SELECT _1.product_name,
		discount_percent
FROM products AS _1
WHERE _1.discount_percent NOT IN
(SELECT _2.discount_percent FROM products AS _2 WHERE _1.product_name <> _2.product_name)
ORDER BY product_name;

-- Problem #6
SELECT DISTINCT email_address, order_id, order_date
FROM customers AS _1 NATURAL JOIN orders
WHERE order_date =
  (SELECT MIN(order_date)
  FROM orders AS _2
  WHERE _1.customer_id = _2.customer_id)