6-2,组合聚集函数
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; COUNT(*) AS num_items,返回字段数目，MIN() AS price_min,prod_price字段的最小值。其他相同
7-1，分组数据
SELECT vend_id, COUNT(*) AS num_prodsFROM Products
GROUP BY vend_id; 分组数据按vend_id分组，结果类似下面：
vend_id num_prods------- ---------BRS01 3DLL01 4
FNG01 2
解释： GROUP BY vend_id，按 vend_id分组，数据库分为了三组，num_prods 各自的数目为3,4,2
注意：GROUP BY 子句必须出现在 WHERE 子句之后， ORDER BY 子句之前
7-2，过滤分组
SELECT cust_id, COUNT(*) AS ordersFROM OrdersGROUP BY cust_id
HAVING COUNT(*) >= 2; 过滤分组有点类似WHERE过滤，这段代码，是按cust_id分组，过滤数目大于等于2的
WHERE与HAVING连用：
SELECT vend_id, COUNT(*) AS num_prodsFROM ProductsWHERE prod_price >= 4GROUP BY vend_id
HAVING COUNT(*) >= 2; 选择prod_price>= 4的且按vend_id>= 2分组，记住COUNT(*)是计数，比如实际应用，可以是
统计价格大于等于4元的，且销售的电脑品牌数量在两台以上的。 比如销售宏碁3台，戴尔销售1台，神舟销售2台，三个品牌价格都大于四元，那样就会筛选出两条数据来。宏碁与神舟。
