1、单表查询的语法

SELECT 字段1,字段2... FROM 表名
　　WHERE 条件
　　GROUP BY field
　　HAVING 筛选
　　ORDER BY field
　　LIMIT 限制条数

2、关键字的执行优先级（重点）

重点中的重点：关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit

1.找到表:from
2.拿着where指定的约束条件，去文件/表中取出一条条记录
3.将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组
4.将分组的结果进行having过滤
5.执行select
6.去重distinct
7.将结果按条件排序：order by
8.限制结果的显示条数

创建公司员工表，表的字段和数据类型

company.employee

    员工id          id                          int

    姓名            name                        varchar

    性别            sex                         enum

    年龄            age                         int

    入职日期         hire_date                  date

    岗位            post                        varchar

    职位描述         post_comment         　　　 varchar

    薪水            salary                　　  double

    办公室           office                     int

    部门编号         depart_id                  int

创建员工表，并插入记录

#创建表,设置字段的约束条件

create table employee(

    id int primary key auto_increment,

    name  varchar(20) not null,

    sex enum('male','female') not null default 'male', #大部分是男的

    age int(3) unsigned not null default 28,

    hire_date date not null,

    post varchar(50),

    post_comment varchar(100),

    salary  double(15,2),

    office int,#一个部门一个屋

    depart_id int

);

# 查看表结构

mysql> desc employee;

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

| Field        | Type                  | Null | Key | Default | Extra          |

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

| id           | int(11)               | NO   | PRI | NULL    | auto_increment |

| emp_name     | varchar(20)           | NO   |     | NULL    |                |

| sex          | enum('male','female') | NO   |     | male    |                |

| age          | int(3) unsigned       | NO   |     | 28      |                |

| hire_date    | date                  | NO   |     | NULL    |                |

| post         | varchar(50)           | YES  |     | NULL    |                |

| post_comment | varchar(100)          | YES  |     | NULL    |                |

| salart       | double(15,2)          | YES  |     | NULL    |                |

| office       | int(11)               | YES  |     | NULL    |                |

| depart_id    | int(11)               | YES  |     | NULL    |                |

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

#三个部门：教学，销售，运营

insert into employee(name ,sex,age,hire_date,post,salary,office,depart_id) values

('mike','male',28,'','teacher',7300.33,401,1), #以下是教学部

('jack','male',30,'','teacher',1000000.31,401,1),

('tom','male',21,'','teacher',8300,401,1),

('lucy','male',25,'','teacher',3500,401,1),

('alice','male',28,'','teacher',2100,401,1),

('frank','female',18,'','teacher',2000,401,1),

('lilei','male',28,'','teacher',30000,401,1),

('hanmeimei','female',28,'','teacher',10000,401,1),

('小美','female',48,'','sale',3000.13,402,2),#以下是销售部门

('小花','female',38,'','sale',2000.35,402,2),

('小草','female',18,'','sale',1000.37,402,2),

('小星','female',18,'','sale',3000.29,402,2),

('小月','female',28,'','sale',4000.33,402,2),    

('程咬金','male',18,'','operation',20000,403,3),#以下是运营部门

('程咬银','female',18,'','operation',19000,403,3),

('程咬铜','male',18,'','operation',18000,403,3),

('程咬铁','female',18,'','operation',17000,403,3);

# 查看表数据

mysql> select * from employee;

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

| id | name      | sex    | age | hire_date  | post      | post_comment | salary     | office | depart_id |

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

|  1 | mike      | male   |  28 | 2012-03-01 | teacher   | NULL         |    7300.33 |    401 |         1 |

|  2 | jack      | male   |  30 | 2015-03-02 | teacher   | NULL         | 1000000.31 |    401 |         1 |

|  3 | tom       | male   |  21 | 2013-03-05 | teacher   | NULL         |    8300.00 |    401 |         1 |

|  4 | lucy      | male   |  25 | 2014-07-01 | teacher   | NULL         |    3500.00 |    401 |         1 |

|  5 | alice     | male   |  28 | 2012-11-01 | teacher   | NULL         |    2100.00 |    401 |         1 |

|  6 | frank     | female |  18 | 2018-02-11 | teacher   | NULL         |    2000.00 |    401 |         1 |

|  7 | lilei     | male   |  28 | 2012-03-01 | teacher   | NULL         |   30000.00 |    401 |         1 |

|  8 | hanmeimei | female |  28 | 2010-11-11 | teacher   | NULL         |   10000.00 |    401 |         1 |

|  9 | 小美      | female |  48 | 2015-03-11 | sale      | NULL         |    3000.13 |    402 |         2 |

| 10 | 小花      | female |  38 | 2010-11-01 | sale      | NULL         |    2000.35 |    402 |         2 |

| 11 | 小草      | female |  18 | 2011-03-12 | sale      | NULL         |    1000.37 |    402 |         2 |

| 12 | 小星      | female |  18 | 2016-05-13 | sale      | NULL         |    3000.29 |    402 |         2 |

| 13 | 小月      | female |  28 | 2017-01-27 | sale      | NULL         |    4000.33 |    402 |         2 |

| 14 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         |   20000.00 |    403 |         3 |

| 15 | 程咬银    | female |  18 | 2013-03-11 | operation | NULL         |   19000.00 |    403 |         3 |

| 16 | 程咬铜    | male   |  18 | 2015-04-11 | operation | NULL         |   18000.00 |    403 |         3 |

| 17 | 程咬铁    | female |  18 | 2014-05-12 | operation | NULL         |   17000.00 |    403 |         3 |

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

3、where 约束　

where子句中可以使用
　　1.比较运算符：>、<、>=、<=、<>、!=
　　2.between 80 and 100 ：值在80到100之间
　　3.in(10,20,30)值是10或20或30
　　4.like 'tom&' line 'tom_': %表示任意多个字符，_表示任意一个字符
　　5.逻辑运算符：多个条件可以使用逻辑运算符 and or not

验证结果：

1.单条件查询

#查询id>5的所有id和name

mysql> select id, name from employee where id>5;

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

| id | name      |

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

|  6 | frank     |

|  7 | lilei     |

|  8 | hanmeimei |

|  9 | 小美      |

| 10 | 小花      |

| 11 | 小草      |

| 12 | 小星      |

| 13 | 小月      |

| 14 | 程咬金    |

| 15 | 程咬银    |

| 16 | 程咬铜    |

| 17 | 程咬铁    |

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

2.多条件查询

#查询职位是teacher，并且薪资大于10000的所有name

mysql> select name from employee where post='teacher' and salary>10000;

+-------+

| name  |

+-------+

| jack  |

| lilei |

+-------+

3.关键字between、and

#查询薪资在10000到20000之间的所有name

mysql> select name, salary from employee where salary between 10000 and 20000;

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

| name      | salary   |

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

| hanmeimei | 10000.00 |

| 程咬金    | 20000.00 |

| 程咬银    | 19000.00 |

| 程咬铜    | 18000.00 |

| 程咬铁    | 17000.00 |

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

#注意''是空字符串，不是null，所以查询post_comment是''时并不能查询到结果

mysql> select name, post_comment from employee where post_comment='';

Empty set (0.00 sec)

执行update再查询就有结果了

mysql> update employee set post_comment='' where id=2;

Query OK, 1 row affected (0.02 sec)

Rows matched: 1  Changed: 1  Warnings: 0

mysql> select name, post_comment from employee where post_comment='';

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

| name | post_comment |

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

| jack |              |

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

4.关键字in、not in集合查询

mysql> select name, salary from employee where salary=10000 or salary=20000 or salary=30000;

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

| name      | salary   |

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

| lilei     | 30000.00 |

| hanmeimei | 10000.00 |

| 程咬金    | 20000.00 |

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

mysql> select name, salary from employee where salary in (10000,20000,30000);

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

| name      | salary   |

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

| lilei     | 30000.00 |

| hanmeimei | 10000.00 |

| 程咬金    | 20000.00 |

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

mysql> select name, salary from employee where salary not in (10000,20000,30000);

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

| name      | salary     |

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

| mike      |    7300.33 |

| jack      | 1000000.31 |

| tom       |    8300.00 |

| lucy      |    3500.00 |

| alice     |    2100.00 |

| frank     |    2000.00 |

| 小美      |    3000.13 |

| 小花      |    2000.35 |

| 小草      |    1000.37 |

| 小星      |    3000.29 |

| 小月      |    4000.33 |

| 程咬银    |   19000.00 |

| 程咬铜    |   18000.00 |

| 程咬铁    |   17000.00 |

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

5.关键字LIKE模糊查询

%表示任意多个字符，_表示任意一个字符

mysql> select * from employee where name like '%m%';

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

| id | name      | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |

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

|  1 | mike      | male   |  28 | 2012-03-01 | teacher | NULL         |  7300.33 |    401 |         1 |

|  3 | tom       | male   |  21 | 2013-03-05 | teacher | NULL         |  8300.00 |    401 |         1 |

|  8 | hanmeimei | female |  28 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |

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

3 rows in set (0.00 sec)

mysql> select * from employee where name like 'm%';

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

| id | name | sex  | age | hire_date  | post    | post_comment | salary  | office | depart_id |

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

|  1 | mike | male |  28 | 2012-03-01 | teacher | NULL         | 7300.33 |    401 |         1 |

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

1 row in set (0.00 sec)

mysql> select * from employee where name like 'to_';

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

| id | name | sex  | age | hire_date  | post    | post_comment | salary  | office | depart_id |

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

|  3 | tom  | male |  21 | 2013-03-05 | teacher | NULL         | 8300.00 |    401 |         1 |

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

1 row in set (0.00 sec)

练习：

1. 查看岗位是teacher的员工姓名、年龄
2. 查看岗位是teacher且年龄大于25岁的员工姓名、年龄
3. 查看岗位是teacher且薪资在9000-10000范围内的员工姓名、年龄、薪资
4. 查看岗位描述不为NULL的员工信息
5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
7. 查看岗位是teacher且名字是t开头的员工姓名、年薪

对应的sql语句

mysql> select name,age from employee where post='teacher';

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

| name      | age |

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

| mike      |  28 |

| jack      |  30 |

| tom       |  21 |

| lucy      |  25 |

| alice     |  28 |

| frank     |  18 |

| lilei     |  28 |

| hanmeimei |  28 |

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

mysql> select name,age from employee where post='teacher' and age>25;

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

| name      | age |

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

| mike      |  28 |

| jack      |  30 |

| alice     |  28 |

| lilei     |  28 |

| hanmeimei |  28 |

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

mysql> select name,age,salary from employee where post='teacher' and salary between 9000 and 10000;

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

| name      | age | salary   |

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

| hanmeimei |  28 | 10000.00 |

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

mysql> select * from employee where post_comment is not null;

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

| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |

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

|  2 | jack | male |  30 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |

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

mysql> select name,age,salary from employee where salary in (9000, 10000,30000);

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

| name      | age | salary   |

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

| lilei     |  28 | 30000.00 |

| hanmeimei |  28 | 10000.00 |

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

mysql> select name,age,salary from employee where salary not in (9000, 10000,30000);

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

| name      | age | salary     |

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

| mike      |  28 |    7300.33 |

| jack      |  30 | 1000000.31 |

| tom       |  21 |    8300.00 |

| lucy      |  25 |    3500.00 |

| alice     |  28 |    2100.00 |

| frank     |  18 |    2000.00 |

| 小美      |  48 |    3000.13 |

| 小花      |  38 |    2000.35 |

| 小草      |  18 |    1000.37 |

| 小星      |  18 |    3000.29 |

| 小月      |  28 |    4000.33 |

| 程咬金    |  18 |   20000.00 |

| 程咬银    |  18 |   19000.00 |

| 程咬铜    |  18 |   18000.00 |

| 程咬铁    |  18 |   17000.00 |

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

mysql> select name,salary from employee where post='teacher' and name like 't%';

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

| name | salary  |

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

| tom  | 8300.00 |

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

4、group by分组查询

#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的
#2、分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等
#3、为何要分组呢？
取每个部门的最高工资
取每个部门的员工数
取男人数和女人数
小窍门：‘每’这个字后面的字段，就是我们分组的依据

#4、大前提：
可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数

当执行以下sql语句的时候，是以post字段查询了组中的第一条数据，没有任何意义，因为我们现在想查出当前组的多条记录。
#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的
如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY

mysql> set global sql_mode='ONLY_FULL_GROUP_BY';

Query OK, 0 rows affected (0.00 sec)

#查看MySQL 5.7默认的sql_mode如下：

mysql> select @@global.sql_mode;

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

| @@global.sql_mode  |

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

| ONLY_FULL_GROUP_BY |

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

1 row in set (0.00 sec)

mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效

Bye

继续验证通过group by分组之后，只能查看当前字段,如果想查看组内信息，需要借助于聚合函数

mysql> select * from emp group by post;# 报错

ERROR 1054 (42S22): Unknown column 'post' in 'group statement'

mysql>  select post from employee group by post;

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

| post                                    |

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

| operation                               |

| sale                                    |

| teacher                                 |

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

4 rows in set (0.00 sec)

5、聚合函数

max()求最大值
min()求最小值
avg()求平均值
sum() 求和
count() 求总个数

#强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组

# 每个部门有多少个员工

mysql> select post, count(id) from employee group by post;

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

| post      | count(id) |

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

| operation |         4 |

| sale      |         5 |

| teacher   |         8 |

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

# 每个部门的最高薪水

mysql> select post, max(salary) from employee group by post;

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

| post      | max(salary) |

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

| operation |    20000.00 |

| sale      |     4000.33 |

| teacher   |  1000000.31 |

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

# 每个部门的最低薪水

select post,min(salary) from employee group by post;

# 每个部门的平均薪水

select post,avg(salary) from employee group by post;

# 每个部门的所有薪水

select post,sum(age) from employee group by post;

6、having过滤

HAVING与WHERE不一样的地方在于
#！！！执行优先级从高到低：where > group by > having

#1. Where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。
#2. Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数

验证：

mysql> select * from employee where salary>1000000;

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

| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |

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

|  2 | jack | male |  30 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |

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

mysql> select * from employee having salary>1000000;

ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause

# 必须使用group by才能使用group_concat()函数，将所有的name值连接

mysql> select post,group_concat(name) from employee group by post having salary > 10000; ##错误，分组后无法直接取到salary字段

ERROR 1054 (42S22): Unknown column 'post' in 'field list'

小练习：

1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
2. 查询各岗位平均薪资大于10000的岗位名、平均工资
3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资

#1、

mysql> select post, count(id), group_concat(name) from employee group by post;

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

| post      | count(id) | group_concat(name)                             |

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

| operation |         4 | 程咬铁,程咬铜,程咬银,程咬金                     |

| sale      |         5 | 小月,小星,小草,小花,小美                        |

| teacher   |         8 | hanmeimei,lilei,frank,alice,lucy,tom,jack,mike |

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

#2、

mysql> select post, avg(salary)  from employee group by post having avg(salary) >10000;

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

| post      | avg(salary)   |

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

| operation |  18500.000000 |

| teacher   | 132900.080000 |

#3、

mysql> select post, avg(salary)  from employee group by post having avg(salary) between 10000 and 20000;;

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

| post      | avg(salary)  |

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

| operation | 18500.000000 |

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

7、order by查询排序

按单列排序 asc升序（默认）， desc(降序）

SELECT * FROM employee ORDER BY age;
SELECT * FROM employee ORDER BY age ASC;
SELECT * FROM employee ORDER BY age DESC;

按多列排序:先按照age升序排序，如果年纪相同，则按照id降序
SELECT * from employee
　　ORDER BY age ASC,
　　id DESC;

小练习：

1. 查询所有员工信息，先按照age升序排序，如果age相同则按照hire_date降序排序
2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列　

#1、

mysql> select * from employee order by age, hire_date desc;

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

| id | name      | sex    | age | hire_date  | post      | post_comment | salary     | office | depart_id |

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

|  6 | frank     | female |  18 | 2018-02-11 | teacher   | NULL         |    2000.00 |    401 |         1 |

| 12 | 小星      | female |  18 | 2016-05-13 | sale      | NULL         |    3000.29 |    402 |         2 |

| 16 | 程咬铜    | male   |  18 | 2015-04-11 | operation | NULL         |   18000.00 |    403 |         3 |

| 17 | 程咬铁    | female |  18 | 2014-05-12 | operation | NULL         |   17000.00 |    403 |         3 |

| 15 | 程咬银    | female |  18 | 2013-03-11 | operation | NULL         |   19000.00 |    403 |         3 |

| 11 | 小草      | female |  18 | 2011-03-12 | sale      | NULL         |    1000.37 |    402 |         2 |

| 14 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         |   20000.00 |    403 |         3 |

|  3 | tom       | male   |  21 | 2013-03-05 | teacher   | NULL         |    8300.00 |    401 |         1 |

|  4 | lucy      | male   |  25 | 2014-07-01 | teacher   | NULL         |    3500.00 |    401 |         1 |

| 13 | 小月      | female |  28 | 2017-01-27 | sale      | NULL         |    4000.33 |    402 |         2 |

|  5 | alice     | male   |  28 | 2012-11-01 | teacher   | NULL         |    2100.00 |    401 |         1 |

|  1 | mike      | male   |  28 | 2012-03-01 | teacher   | NULL         |    7300.33 |    401 |         1 |

|  7 | lilei     | male   |  28 | 2012-03-01 | teacher   | NULL         |   30000.00 |    401 |         1 |

|  8 | hanmeimei | female |  28 | 2010-11-11 | teacher   | NULL         |   10000.00 |    401 |         1 |

|  2 | jack      | male   |  30 | 2015-03-02 | teacher   |              | 1000000.31 |    401 |         1 |

| 10 | 小花      | female |  38 | 2010-11-01 | sale      | NULL         |    2000.35 |    402 |         2 |

|  9 | 小美      | female |  48 | 2015-03-11 | sale      | NULL         |    3000.13 |    402 |         2 |

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

17 rows in set (0.00 sec)

#2、

mysql> select post, avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary);

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

| post      | avg(salary)   |

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

| operation |  18500.000000 |

| teacher   | 132900.080000 |

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

#3、

mysql> select post, avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) desc;

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

| post      | avg(salary)   |

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

| teacher   | 132900.080000 |

| operation |  18500.000000 |

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

8、limit限制查询的记录数量

示例：
SELECT * FROM employee ORDER BY salary DESC LIMIT 3; #默认初始位置为0

SELECT * FROM employee ORDER BY salary DESC LIMIT 0,5; #从第0开始，即先查询出第一条，然后包含这一条在内往后查5条

SELECT * FROM employee ORDER BY salary DESC LIMIT 5,5; #从第5开始，即先查询出第6条，然后包含这一条在内往后查5条

小练习：
分页显示，每页5条

mysql> select * from employee limit 0,5;

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

| id | name  | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |

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

|  1 | mike  | male |  28 | 2012-03-01 | teacher | NULL         |    7300.33 |    401 |         1 |

|  2 | jack  | male |  30 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |

|  3 | tom   | male |  21 | 2013-03-05 | teacher | NULL         |    8300.00 |    401 |         1 |

|  4 | lucy  | male |  25 | 2014-07-01 | teacher | NULL         |    3500.00 |    401 |         1 |

|  5 | alice | male |  28 | 2012-11-01 | teacher | NULL         |    2100.00 |    401 |         1 |

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

5 rows in set (0.05 sec)

mysql> select * from employee limit 5,5;

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

| id | name      | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |

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

|  6 | frank     | female |  18 | 2018-02-11 | teacher | NULL         |  2000.00 |    401 |         1 |

|  7 | lilei     | male   |  28 | 2012-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |

|  8 | hanmeimei | female |  28 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |

|  9 | 小美      | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |

| 10 | 小花      | female |  38 | 2010-11-01 | sale    | NULL         |  2000.35 |    402 |         2 |

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

5 rows in set (0.00 sec)

mysql> select * from employee limit 10,5;

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

| id | name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |

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

| 11 | 小草      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |

| 12 | 小星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |

| 13 | 小月      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |

| 14 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |

| 15 | 程咬银    | female |  18 | 2013-03-11 | operation | NULL         | 19000.00 |    403 |         3 |

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

5 rows in set (0.00 sec)

mysql> select * from employee limit 15,5;

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

| id | name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |

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

| 16 | 程咬铜    | male   |  18 | 2015-04-11 | operation | NULL         | 18000.00 |    403 |         3 |

| 17 | 程咬铁    | female |  18 | 2014-05-12 | operation | NULL         | 17000.00 |    403 |         3 |

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

2 rows in set (0.00 sec)

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