1、数据操作

(1)insert 增加数据

1. 插入完整数据(顺序插入)

语法一：

INSERT INTO 表名(字段1,字段2,字段3…字段n) VALUES(值1,值2,值3…值n);

语法二：

INSERT INTO 表名 VALUES (值1,值2,值3…值n);

2. 指定字段插入数据

语法：

INSERT INTO 表名(字段1,字段2,字段3…) VALUES (值1,值2,值3…);

3. 插入多条记录

语法：

INSERT INTO 表名 VALUES (值1,值2,值3…值n), (值1,值2,值3…值n), (值1,值2,值3…值n);

4. 插入查询结果

语法：

INSERT INTO 表名(字段1,字段2,字段3…字段n)

SELECT (字段1,字段2,字段3…字段n) FROM 表2

WHERE …;

(2)delete 删除数据

语法：

DELETE FROM 表名 WHERE CONITION;

示例：

DELETE FROM mysql.user

WHERE password=’’;

(3)update 修改数据

语法：

UPDATE 表名 SET 字段1=值1, 字段2=值2 WHERE CONDITION;

示例：

UPDATE mysql.user SET password=password(‘123’)

where user=’root’ and host=’localhost’;

(4)select 查询数据

语法

SELECT DISTINCT FROM 表名;

2、单表查询

单表查询的语法

SELECT DISTINCT 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;

单表查询的关键字执行的优先级

from

where

group by

select

distinct

having

order by

limit

单表查询的执行过程

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.执行select(也可以distinct去重)

5.将分组的结果进行having过滤

6.将结果按条件排序：order by

7.限制结果的显示条数

(1)前期建数据库和表

# 建表和数据准备

# 创建表

create table employee(

id int not null unique auto_increment,

emp_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, #一个部门一个屋子

depart_id int

);

# 插入记录

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

#以下是教学部

('cai','male',78,'20150302','teacher',1000000.31,401,1),

('liu','male',81,'20130305','teacher',8300,401,1),

('ling','male',73,'20140701','teacher',3500,401,1),

('yongliang','male',28,'20121101','teacher',2100,401,1),

('marry','female',18,'20110211','teacher',9000,401,1),

('tom','male',18,'19000301','teacher',30000,401,1),

('liangliang','male',48,'20101111','teacher',10000,401,1),

#以下是销售部门

('阿哥','female',48,'20150311','sale',3000.13,402,2),

('丫头','female',38,'20101101','sale',2000.35,402,2),

('小丁','female',18,'20110312','sale',1000.37,402,2),

('月亮','female',18,'20160513','sale',3000.29,402,2),

('太阳','female',28,'20170127','sale',4000.33,402,2),

#以下是运营部门

('老才','male',28,'20160311','operation',10000.13,403,3),

('老李','male',18,'19970312','operation',20000,403,3),

('老王','female',18,'20130311','operation',19000,403,3),

('老德','male',18,'20150411','operation',18000,403,3),

('老赵','female',18,'20140512','operation',17000,403,3)

;

(2)简单查询

# 查看表中所有

select * from 表;

# 指定列查询

select emp_name,salary from employee;

# 在列中使用四则运算

select emp_name,salary*12 from employee;

# 重命名

select emp_name,salary*12 as annul_salary from employee;

select emp_name,salary*12 annul_salary from employee;

# 去重 distinct

select distinct post from employee;

select distinct sex,post from employee;

# 定义显示格式

concat()函数 # 用于拼接字符串

select concat('姓名 ：',emp_name),concat('年薪:',salary*12) from employee;

concat_ws()函数 第一个参数为分隔符

select concat_ws('|','a','b','c');

# 条件判断

case when语句 == if条件判断句

select

(

case

when emp_name = 'cai' then

emp_name

when emp_name = 'liu' then

CONCAT(emp_name,'_haha')

else

concat(emp_name, 'hehe')

end

) as new_name

from

employee;

(3)根据条件筛选行 where

# 比较运算 = > < >= <= !=/<>

select * from employee where age>18;

select * from employee where salary<10000;

select * from employee where salary=20000;

# between 在一个范围内； between a and b #[a,b]

select * from employee where salary between 10000 and 20000;

# in集合查询 ； in(80,90,100) 值是80或90或100

select * from employee where salary in (17000,19000);

# like 模糊查询

# _ 通配符 表示一个字符长度的任意内容

select * from employee where emp_name like 'c__';

# % 通配符 表示任意字符长度的任意内容

select * from employee where emp_name like 'c%';

select * from employee where emp_name like '%u';

select * from employee where emp_name like '%a%';

# regexp 正则匹配

select * from employee where emp_name regexp '^cai';

(4)逻辑运算 and or not

# and

select * from employee where age>18 and post='teacher';

# or

select * from employee where salary<10000 or salary>30000;

# not

select * from employee where salary not in (10000,17000,18000);

(5)关于 null

# 关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS)

# 查看岗位描述为NULL的员工信息

select * from employee where post_comment is null;

# 查看岗位描述不为NULL的员工信息

select * from employee where post_comment is not null;

(6)5个聚合函数

​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

count # 统计次数

max # 求最大值

min # 求最小值

avg # 求平均值

sum # 求和

(7)分组聚合 group by

​如果我们用unique的字段作为分组的依据，则每一条记录自成一组，这种分组没有意义

​多条记录之间的某个字段值相同，该字段通常用来作为分组的依据

# 单独使用group by关键字分组

select post from employee group by post;

#注意：我们按照post字段分组，那么select查询的字段只能是post，想要获取组内的其他相关信息，需要借助函数

#group by关键字和group_concat()函数一起使用

#按照岗位分组，并查看组内成员名

select post,group_concat(emp_name) from employee group by post;

select post,group_concat(emp_name) as emp_members from employee group by post;

# group by与聚合函数一起使用

#按照岗位分组，并查看每个组有多少人

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

# 查询各部门年龄在20岁以上的人的平均薪资

select post,avg(salary) from employee where age>20 group by post;

(8)过滤 having (group by + 聚合函数)

# 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

# 查询平均薪资大于1w的部门

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

# 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

select post,group_concat(emp_name),count(id) from employee group by post having count(id)<2;

# 查询各岗位平均薪资大于10000的岗位名、平均工资

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

# 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资

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

(9)排序 order by

# 升序 asc (默认升序)

select * from employee order by salary;

select * from employee order by salary asc;

# 降序 desc

select * from employee order by salary desc;

# 按多列排序:先按照age排序，如果年纪相同，则按照薪资排序

select * from employee order by age,salary;

select * from employee order by age,salary desc;

select * from employee order by age desc,salary;

(10)限制查询的记录数 limit

# 取前n条

select * from 表 order by 列 limit n;

# 从m+1开始,取n条

select * from 表 order by 列 limit m,n;

# 从m+1开始,取n条

select * from 表 order by 列 limit n offset m;

小结：select * from 表 where 条件 group by 分组 having 过滤 order by 排序 limit n;

3、在命令提示符窗口中遇到的编码问题解决方法：

1.临时解决问题 在客户端执行 set xxxx = utf8;

2.永久解决问题 在my.ini添加 set xxxx = utf8;

3.实时解决问题 create table 表名() charset=utf8;