聚集函数以及如何利用聚集函数汇总数据
聚集函数运行在行组上，返回单个函数值
AVG()函数： 只能确定特定数列之的平均值，并且会忽略NULL值
select avg(prod_price) as avg_price from products;
select prod_price,prod_name,prod_id from products;
select sum(prod_price) as sum_price from products;
select prod_price,prod_name,prod_id from products where vend_id =1003;
select avg(prod_price) as avg_price from products where vend_id =1003;
select sum(prod_price) as sum_price from products where vend_id =1003;
当使用聚集函数时，不能进行筛选操作，会报错。因为聚集函数返回的时单个的函数值，而筛选操作返回的时一系列行
COUNT()函数： 计数函数
select count(*) as num_cust from customers;
select cust_name from customers;
select count(cust_email) as num_cust from customers;
select cust_email from customers;
这里对cust_email进行计数，count()函数会忽略null值，如果是count(*)则不会忽略NULL值
MAX（）函数： 返回指定列最大值 忽略NULL值
MIN（）函数：功能同上
select max(prod_price) as max_price from products;
select prod_price from products;
sum()函数： 求和
select sum(quantity) as items_ordered from orderitems where order_num = 20005;
select quantity from orderitems where order_num = 20005;
select item_price,quantity from orderitems where order_num = 20005;
select sum(quantity*item_price) as total_money from orderitems where order_num = 20005;
聚集不同值
select avg(distinct prod_price) as avg_price from products where vend_id = 1003;
指定供货商1003，只考虑不同价格distinct prod_price
