使用pymysql 单表查询 单标查询语法: 关键字执行的优先级 建表: Where查询: GROUP BY 分组查询: HAVING 组过滤: ORDER BY 排序查询:LIMIT限制查询:

day 40

一丶使用pymysql

1.下载pymysql包: pip3 install pymysql

2.编写代码

###### 增加数据 
import pymysql      # 导入pymysql模块
​
# 1. 生成conn连接对象 固定语法
conn=pymysql.Connect(host='127.0.0.1', user='root', password="123",
                 database='day40', port=3306,)
​
# 2.创建游标对象,用于操作数据库
cur=conn.cursor()
​
with open('test','r',encoding='utf-8') as f:
    for line in f:
        # 学python从开始到放弃 | alex | 人民大学出版社 | 50 | 2018 - 7 - 1
        line=line.strip().split('|')
        # 3 编写sql语句  %s是sql语言的占位符 和python无关
        sql='insert into book(b_name,auth, press,price,p_date) values(%s,%s,%s,%s,%s)'
        # 4. 执行sql语句 , execute方法,可以拼接sql语句
        cur.execute(sql,line)
        # 5.  增, 删, 改 数据 都需要进行提交
        conn.commit()
​
# 6 关闭操作游标, 关闭连接
cur.close()
conn.close()

#### 查询数据
​
import pymysql      # 导入pymysql模块
​
# 1. 生成conn连接对象 固定语法
conn=pymysql.Connect(host='127.0.0.1', user='root', password="123",
                 database='day40', port=3306,)
​
# 2.创建游标对象,用于操作数据库
cur=conn.cursor()
​
# 3. 编写sql语句
sql = 'select * from book'
# 4. 执行sql
cur.execute(sql)
​
# 5. 查询结果
res1 = cur.fetchone()  # 查询一条  以元组的形式返回数据
res2 = cur.fetchmany(2)  # 查询多条  
res3 = cur.fetchall()  # 查询全部
​
print(res1)     # 控制台打印结果
print(res2)
print(res3)
​
# 6 关闭操作游标, 关闭连接
cur.close()
conn.close()

 

 

 

二丶单表查询

单标查询语法:

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

关键字执行的优先级

#3## 特别重要 👇
    from         :找到表:from
    where         :拿着where指定的约束条件，去文件/表中取出一条条记录
    group by      :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
    select        :执行select
    distinct      :去重
    having        :将分组的结果进行having过滤
    order by      : 将结果按条件排序：order by    desc降序  acs升序
    limit         :限制结果的显示条数

 

建表:

# 创建表
company.employee
    员工id      id                  int             
    姓名        emp_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
);
​
​
#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    |                |
| salary       | double(15,2)          | YES  |     | NULL    |                |
| office       | int(11)               | YES  |     | NULL    |                |
| depart_id    | int(11)               | YES  |     | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+
​
#插入记录
#三个部门：教学，销售，运营
insert into employee(emp_name,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('成龙','male',48,'20101111','teacher',10000,401,1),
​
('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),
​
('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;
​
#ps：如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk

 

简单查询:select

### 使用函数
    select user();      # 获取当前用户
    select database();  # 获取当前数据库
    select now();       # 获取当前时间
    
    
### 简单查询
    # 查询全部字段
    SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;
    
    # 查所有
    SELECT * FROM employee;
    
    # 查询指定字段
    SELECT emp_name,salary FROM employee;
​
​
### 查询去重 distinct
    # 查看有几个部门
    select distinct post from employee;
​
​
### 查询通过四则运算
    # 查看 每个人的年薪
    select emp_name, salary*12 from emplpoyee;
    
    
### 更改别名
    # as 
    select emp_name, salary*12 AS Annual_salary from employee;
    
    # 字段后直接跟别名
    select emp_name, salary*12  Annual_salary from employee;
    
    
### 定义显示格式 
    # CONCAT()函数: 字段与分隔符号 以逗号间隔
    select concat('姓名:',emp_name,'年薪:',salary*12) from employee;
    
    # CONCAT_WS()函数: 第一个参数必须是分隔符,
    select concat_ws(':' , emp_name, salary*12) from employee;
    
    # 结合CASE 语句:
    #语法: ( case   when 条件1 then 显示内容    when 条件2 then 显示内容   else  显示内容  end )
    
    select 
    (
        case
        when emp_name='jingliyang' then
            emp_name
        when emp_name='alex' then
            concat(emp_name,'_BIGSB')
        else
            concat(emp_name,'SB')
        end 
    ) as new_name
    from 
        employee;
        
        
#### 练习:
#1 查出所有员工的名字，薪资,格式为
    <名字:egon>    <薪资:3000>
    # select concat('<名字:',emp_name,'>'),concat('<薪资:',salary,'>') from employee;
​
#2 查出所有的岗位（去掉重复）
    # select distinct post from employee;
#3 查出所有员工名字，以及他们的年薪,年薪的字段名为annual_year 
    # select emp_name ,salary*12 from employee;

Where查询:

###  常用的模式
​
# 比较运算符:  > 大于,  < 小于,  >= 大于等于,  <=小于等于 , != 不等于, <> 不等于
​
# between  80  and 100   : 在80 到 100 的范围内, 包含 80 和 100
​
# in(80,90,100) : 值是80 或者 90 或者 100
​
# 模糊查询
    # like : % 表示任意多个字符, _表示一个字符
        # 1.  like 'a%'   以a开头的.
        # 2.  like '%a'   以a结尾的
        # 3.  like '_a'   Xa 两个字符
        # 4.  like 'a_'   aX 两个字符
      
     # regexp :正则匹配
         # 1. '^a' 以a开头
         # 2. '\d+' 纯数字
         # 3. 'a$' 以a结尾
         
         
#  is  和 is not
    is null : 是空
    is not null  : 非空
    
​
# 逻辑运算符:  and与  or或   not非
​
​

where案例

# 1. 条件查询
    select emp_name from employee where post='sale';
​
# 2. 多条件查询
    select emp_name , salary from employee where port='teacher' and salary>10000;
​
# 3.关键字 between  and
    select emp_name ,salary from employee where salary beetween 10000 and 20000;
​
# 4.关键字 is null  判断某个字段是不是空,不能用等号
    select emp_name,post_comment from employee where post_comment is null;
    select emp_name,post_comment from employee where post_comment is not null;
    # 判断是不是
    select emp_name,post_comment from employee where post_comment='' ;
​
# 5.关键字IN集合查询
    SELECT emp_name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT emp_name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;
​
    SELECT emp_name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;
        
# 6.关键字LIKE模糊查询
    通配符 '%' 多个字符
        # 查询eg开头
        select * from employee
              where emp_name like 'eg%';
    通配符 '_' 单个字符
        # select * from employee 
            where emp_name like 'al__';
​
# 7 . regexp 依据正则匹配数据
    # 找到以jin开头的数据   
    select emp_name  ,salary*12 from employee where emp_name regexp '^jin'
    
    
### 练习
1. 查看岗位是teacher的员工姓名、年龄
    # select emp_name , age from employee where post ='teacher';
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
     # select emp_name , age from employee where post ='teacher' and age>30;
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
    # select emp_name , age  ,salary*12  from employee where post ='teacher' and  salary between 9000 and 10000;
4. 查看岗位描述不为NULL的员工信息
    #  select * from employee where post_comment is not null;
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
    # select emp_name , age  ,salary from employee where post ='teacher' and  salary in (9000,10000,30000);
    #  select emp_name , age  ,salary from employee where post ='teacher' and  salary=9000 or salary=10000 or salary=30000;
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
    #  select emp_name , age  ,salary from employee where post ='teacher' and  salary not in (9000,10000,30000);
    #  select emp_name , age  ,salary from employee where post ='teacher' and  not (salary=9000 or salary=10000 or salary=30000);
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪
    # select emp_name ,salary*12 from employee where post='teacher' and emp_name like 'jin%';
    # select emp_name ,salary*12 from employee where post='teacher' and emp_name regexp '^jin';

 

GROUP BY 分组查询:

特点:

根据某个重复率比较高的字段进行的

一旦分组了就不能对具体的一条数据进行操作

group_concat():只用来做最重的显示,不能作为中间的结果操作其他数据

分组具有去重的效果

### 单独使用group by 关键字分组查询 . 每次操作都是以组的形式操作这些数据
	# 按照部门进行分组,获得每个部门的名字 (有去重的效果)
	select post from employee group by post;

### group by 和 group_concat()函数一起使用
	# group_concat()只是用来显示内容
	# 按照岗位分组,并查看组内所有成员
	select post,group_concat(emp_name) from employee group by post; 
	

### group by 和 聚合函数 一起使用
	# 按照岗位分组,并查看每组有多少人
	select post,count(id) as count for employee group by post;
	

##### 强调
	1.如果我使用 unique 字段作为分组依据, 则每条记录自成一组,这样没有意义
	2. 通常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
	

 

HAVING 组过滤:

特点:

对一个组进行条件筛选

优先级: 

where > group by > having

1.where发生在分组group by之前,where可以有任意字段,where绝对不会和聚合函数一起使用.

2.having发生在分组group by之后,因而having可以使用分组的字段,单无法直接取到其他字段

## 验证 1.
	mysql> select post from employee where count(salary) group by post;
	ERROR 1111 (HY000): Invalid use of group function

## 验证 2.
	#错误，分组后无法直接取到salary字段
	mysql> select post,group_concat(emp_name) from employee group by post having salary>1000;
	ERROR 1054 (42S22): Unknown column 'salary' in 'having clause'
	
	#可以使用聚合函数获取未定义字段信息
	mysql> select post,group_concat(emp_name) from employee group by post having avg(salary)>10000;
    +-----------+---------------------------------------------------------+
    | post      | group_concat(emp_name)                                  |
    +-----------+---------------------------------------------------------+
    | operation | 程咬铁,程咬铜,程咬银,程咬金,张野                        |
    | teacher   | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex   |
    +-----------+---------------------------------------------------------+
	
	
	
### 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
    	# select post, count(id),group_concat(emp_name) from employee  group by post having count(id)<2;
    3. 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;
    4. 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)>10000 and avg(salary)<20000;
    	#  select post,avg(salary) from employee group by post having avg(salary) between 10000 and 20000;

 

聚合函数:

count() :统计某个字段出现的次数

max() : 某个字段的最大值

min() : 某个字段的最小值

avg() : 某个字段的平均值

sum() : 某个字段进行求和

### 如果没有进行分组,  那么这张表会作为一个整体成为一组

### 应用实例
	# 统计这张表有多少条记录
	select count(*) from employee;
	# 统计这个表id字段有效值有多少个.(有效值 指的是非空)
	select count(id) from employee;
	
	# 最高的工资
	select max(salary) from employee;
	# 最低的工资
	select min(salary) from employee;
	# 平均工资
	select avg(salary) from employee;
	# 工资总和
	select sum(salary) from employee;
	
	
### 练习
	1. 查询岗位名以及岗位包含的所有员工名字
	# select post, group_concat(emp_name) from employee group by post;
	2. 查询岗位名以及各岗位内包含的员工个数
	# select post, count(emp_name) from employee group by post;
	3. 查询公司内男员工和女员工的个数
	# select count(emp_name),sex from employee group by sex;
	4. 查询岗位名以及各岗位的平均薪资
	# select post, avg(salary) from employee group by post ;
	5. 查询岗位名以及各岗位的最高薪资
	# select post, max(salary) from employee group by post ;
	6. 查询岗位名以及各岗位的最低薪资
	# select post, min(salary) from employee group by post ;
	7. 查询男员工与男员工的平均薪资，女员工与女员工的平均薪资
	# select sex, max(salary) from employee group by sex ;

    # 求各部门薪资大于1w的人的个数
    select post,count(id) from employee where salary >10000 group by post;

 

ORDER BY 排序查询:

特点:

1.对单字段, 对多字段进行排序

2.默认升序 从小到大

3.desc 降序 从大到小, asc 升序 从小到大

### 按单列排序
	# 升序排序工资
	select * from employee order by salary;
	# asc 升序
	select * from employee order by salary asc;
	# desc 降序
	select * from employee order by salary desc;	
	
### 按多列排序: 
	# 先排age 升序 ,再排 薪资 升序
	select * from employee order by age , salary ;
	
	# 先排age 降序 ,再排 薪资 升序
	select * from employee order by age desc , salary ;



### 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
    # select * from employee order by age , hire_date desc;
    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
    # select post , avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary);
    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
    # select post , avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) desc;

 

LIMIT限制查询:

特点:

1.limit(n,m) : n默认从0开始 , 从n+1开始, 取m条

2.与 limit n offset m : 用法一直

3.limit(n) : 取n条

4.和order by 搭配使用

应用:

1.分页

2.限制取值

## 实例
	# 降序排序工资,每次取 3 条
	select * from employee order by salary desc limit(3);
	
	# 从第1条开始,即查出第一条,包含这一条并继续查询5条
	select * from employee order by salary desc limit 0,5;
	
	# 从第6条开始,即查出第6条,包含这一条并继续查询5条
	select * from employee order by salary desc limit 5,5;
	
	
### 分页练习 分页显示，每页5条

	mysql> select * from employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | emp_name  | sex  | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
|  1 | egon      | male |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
|  2 | alex      | male |  78 | 2015-03-02 | teacher                                 | NULL         | 1000000.31 |    401 |         1 |
|  3 | wupeiqi   | male |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
|  4 | yuanhao   | male |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  5 | liwenzhou | male |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+


	mysql> select * from employee limit 5,5;
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | emp_name   | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
|  8 | 成龙       | male   |  48 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale    | NULL         |  2000.35 |    402 |         2 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec)

 

正则查询:

特点:

mysql可以使用正则进行查询

## 实例
	# emp_name字段以al开头的数据
	select * from employee where emp_name regexp '^al';
	
	# emp_name字段以on结尾的数据
	select * from employee where emp_name regexp 'on$';
	
	# emp_name字段以al开头的数据
	select * from employee where emp_name regexp '^al';