汇总数据
聚集函数：运行在行组上，计算和返回单个值的函数
1）AVG()函数：AVG()通过对表中行数计算并计算特定列值之和，求得该列的平均值。AVG()可用来返回所有列的平均值，也可以用来返回特定列或行的平均值
返回products表中所有产品的平均价格
SELECT AVG(prodprice) AS avgprice
FROM products;
返回特定供应商所提供产品的平均价格
SELECT AVG(prodprice) AS avgprice
FROM products
WHERE vend_id =1003;
2)COUNT()函数
返回customers表中客户的总数：
SELECT COUNT(*) AS num_cust
FROM customers;
只对具有电子邮件地址的客户计数
SELECT COUNT(custemail) AS numcust
FROM customers;
3)MAX()函数：要求指定列名
SELECT MAX(prodprice) AS maxprice
FROM products;
4)MIN()函数：要求指定列名
SELECT MIN(prodprice) AS minprice
FROM products;
5)SUM()函数：用来返回指定列值的和（总计）
检索所订购物品的总数（所有quantity值之和）
SELECT SUM(quantity) AS items_ordered
FROM orderitems
WHERE order_num=20005;
合计每项物品的items_pricequantity，得到总的订单金额
SELECT SUM(itempricequantity) AS totalprice
FROM orderitems
WHERE order_num=20005;
6)聚集不同值
7）组合聚集函数
SELECT COUNT(*) AS num_items,
MIN(prodprice) AS pricemin,
MAX(prodprice) AS pricemax,
AVG(prodprice) AS priceavg
FROM products;
