聚合函数

1. select avg(prod_price) AS avg_price from products;
2. select count(*) as num_cut from customers;
3. select max(prod_price) AS max_price from products;
4. select min(prod_price) AS max_price from products;
5. select sum(quantity) AS items_ordered from orderitems where order_num = 20005;
6. select sum(quantity * price) AS total_price from orderitems where order_num = 20005;