20.使用聚合函数查询（单独一篇）
mysql> select count(*) as cust_num
    -> from customers;
+----------+
| cust_num |
+----------+
|        3 |
+----------+
1 row in set

mysql> select count(c_email) as email_num
    -> from customers;
+-----------+
| email_num |
+-----------+
|         2 |
+-----------+
1 row in set

mysql> select f_name,count(f_id)
    -> from fruits
    -> group by f_name
    -> ;
+--------+-------------+
| f_name | count(f_id) |
+--------+-------------+
| apple  |           1 |
| banana |           1 |
| caomei |           1 |
+--------+-------------+
3 rows in set



mysql> create table orderitems
    -> (
    -> o_num    int not null,
    -> o_item   int not null,
    -> f_id char(10) not null,
    -> quantity int not null,
    -> item_price decimal(8,2) not null,
    -> primary key (o_num,o_item)
    -> );
Query OK, 0 rows affected
mysql> insert into orderitems
    -> values(1001,1,"a1",10,5.2),
    -> (1002,2,"b1",11,6.2),#逗号间隔不可少。
    -> (1003,3,"c1",12,7.2);
Query OK, 3 rows affected
Records: 3  Duplicates: 0  Warnings: 0

mysql> select o_num,sum(quantity) as items_total
    -> from orderitems
    -> group by o_num;
+-------+-------------+
| o_num | items_total |
+-------+-------------+
|  1001 | 10          |
|  1002 | 11          |
|  1003 | 12          |
+-------+-------------+
3 rows in set

mysql> select avg(f_price) as avg_price
    -> from fruits
    -> where s_id = 103;
+-----------+
| avg_price |
+-----------+
|       7.2 |
+-----------+
1 row in set

mysql> select s_id,avg(f_price) as avg_price
    -> from fruits
    -> group by s_id;
+------+-----------+
| s_id | avg_price |
+------+-----------+
|  101 |       5.2 |
|  102 |       6.2 |
|  103 |       7.2 |
+------+-----------+
3 rows in set

mysql> select max(f_price) as max_price from fruits;
+-----------+
| max_price |
+-----------+
| 7.2       |
+-----------+
1 row in set

mysql> select s_id,max(f_price) as max_price
    -> from fruits
    -> group by s_id;
+------+-----------+
| s_id | max_price |
+------+-----------+
|  101 | 5.2       |
|  102 | 6.2       |
|  103 | 7.2       |
+------+-----------+
3 rows in set

mysql> select max(f_name) from fruits;
+-------------+
| max(f_name) |
+-------------+
| caomei      |
+-------------+
1 row in set

mysql> select min(f_price) as min_price from fruits;
+-----------+
| min_price |
+-----------+
| 5.2       |
+-----------+
1 row in set

mysql> select s_id,min(f_price) as min_price
    -> from fruits
    -> group by s_id;
+------+-----------+
| s_id | min_price |
+------+-----------+
|  101 | 5.2       |
|  102 | 6.2       |
|  103 | 7.2       |
+------+-----------+
3 rows in set