Python进阶----pymysql模块的使用,单表查询

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:

​   ​   ​   ​   ​ 1. 优先级where > group by > having

​   ​   ​   ​   ​2.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.

   ​   ​   ​   3.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';

转载于:https://www.cnblogs.com/dengl/p/11290953.html