样板，空值(null)自改

create database sky;

use sky;

create table m1(

id int(11),

name char(20),

age tinyint(10),

sex enum('男','女'),

score tinyint(10),

address char(20)

) default charset=utf8;

insert into m1 values

(1,'L1',21,'男',90,'北京'),

(2,'L2',19,'男',91,'上海'),

(3,'L3',24,'女',95,'广州'),

(4,'L4',22,'男',89,'广州'),

(5,'L5',20,'女',86,'上海'),

(6,'L6',19,'女',99,'广州');


分类

1. avg(字段名)：字段的平均值

2. sum(字段名):  字段的和

3. max(字段名):  求字段的最大值

4. min(字段名) ; 求字段的最小值

5. count(字段名): 统计该字段有记录的个数


例如：

mysql> select * from m1;

+------+------+------+------+-------+---------+

| id   | name | age  | sex  | score | address |

+------+------+------+------+-------+---------+

|    2 | L2   | NULL | 男   |    91 | 上海    |

|    3 | L3   |   24 | 女   |    95 | 广州    |

|    4 | L4   |   22 | 男   |   101 | 广州    |

|    5 | L5   | NULL | 女   |    86 | 上海    |

|    6 | L6   |   19 | 女   |    99 | 北京    |

|    1 | L1   |   21 | 男   |    90 | 北京    |

+------+------+------+------+-------+---------+

6 rows in set (0.00 sec)


显示score字段最高的

mysql> select max(score) as SCORE from m1;     # as 别名

+-------+

| SCORE |

+-------+

|   101 |

+-------+

1 row in set (0.00 sec)


统计字段有几行,as 别名

mysql> select count(name) as N,count(age) as A from m1;

+---+---+

| N | A |

+---+---+

| 6 | 4 |

+---+---+

1 row in set (0.02 sec)


计算总分

mysql> select sum(score) as score from m1;

+-------+

| score |

+-------+

|   562 |

+-------+

1 row in set (0.01 sec)


计算每个城市的平局分数

mysql> select address,avg(score) from m1 group by address;

+---------+------------+

| address | avg(score) |

+---------+------------+

| 上海    |    88.5000 |

| 北京    |    94.5000 |

| 广州    |    98.0000 |

+---------+------------+

3 rows in set (0.00 sec)


查看那两个国家人最多，显示前两名

mysql> select address,count(name) as name_count from m1

-> group by address

-> order by name_count desc

-> limit 2;

+---------+------------+

| address | name_count |

+---------+------------+

| 广州    |          3 |

| 上海    |          2 |

+---------+------------+

2 rows in set (0.00 sec)


distinct

作用：去重

distinct 处理的是存在的字段名

有多少个城市

mysql> select distinct address from m1;

+---------+

| address |

+---------+

| 北京    |

| 上海    |

| 广州    |

+---------+

3 rows in set (0.00 sec)