SELECT AVG(prod_price) AS avg_price FROM products; #AVG只能用于单个列求平均值，如想计算多个列，必须用多个AVG()

SELECT AVG(prod_price) AS avg_price FROM products

WHERE vend_id = 1003; #AVG函数忽略列值为NULL的行

SELECT COUNT(*) AS num_cust FROM customers; #此时不忽略列为NULL的行

SELECT COUNT(cust_email) AS num_cust FROM customers; #此时忽略列为NULL的行

SELECT MAX(prod_price) AS max_price

FROM products; #忽略列为NULL的行

SELECT MIN(prod_price) AS max_price

FROM products; #忽略列为NULL的行

SELECT SUM(quantity) AS items_ordered FROM orderitems

WHERE order_num = 20005;

SELECT SUM(item_price*quantity) AS total_price FROM orderitems

WHERE order_num = 20005; #忽略列值为NULL的行

SELECT AVG(DISTINCT prod_price) AS avg_price FROM products # 以上5个聚集函数，默认是对所有的行进行计算(ALL)，

WHERE vend_id = 1003; #如果只包含不同的值，可在列名前指定DISTINCT，但DISTINCT不能用于COUNT(*)。

SELECT COUNT(*) AS num_items, MIN(prod_price) AS price_min,

       MAX(prod_price) AS price_max, AVG(prod_price) AS price_avg FROM products;