python3 mysql-单表查询
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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）
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
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
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 | | +--------------+-----------------------+------+-----+---------+----------------+ 10 rows in set (0.08 sec) #插入记录 #三个部门：教学，销售，运营 insert into employee(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), (‘xiaomage‘,‘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) ;
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(80,90,100)值是10或20或30；
4.like ‘xiaomagepattern‘: pattern可以是%或者_。%小时任意多字符，_表示一个字符；
5.逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not。
（1）单条件查询
查询id大于5的员工名和员工id。
mysql> select id,emp_name from employee where id > 5; +----+------------+ | id | emp_name | +----+------------+ | 6 | jingliyang | | 7 | jinxin | | 8 | xiaomage | | 9 | 歪歪 | | 10 | 丫丫 | | 11 | 丁丁 | | 12 | 星星 | | 13 | 格格 | | 14 | 张野 | | 15 | 程咬金 | | 16 | 程咬银 | | 17 | 程咬铜 | | 18 | 程咬铁 |
（2）多条件查询
查询部门是老师且工资大于10000的员工名。
mysql> select emp_name from employee where post=‘teacher‘ and salary>10000; +----------+ | emp_name | +----------+ | alex | | jinxin | +----------+
（3）关键字between and
查找工资在10000到20000之间的员工姓名和工资
查找工资不在10000到20000之间的员工姓名和工资
SELECT name,salary FROM employee WHERE salary BETWEEN 10000 AND 20000; SELECT name,salary FROM employee WHERE salary NOT BETWEEN 10000 AND 20000;
（4）关键字in集合查询
查询工资是3000、3500、4000、9000的员工姓名和工资。
mysql> SELECT name,salary FROM employee WHERE salary IN (3000,3500,4000,9000) ; +------------+---------+ | name | salary | +------------+---------+ | yuanhao | 3500.00 | | jingliyang | 9000.00 | +------------+---------+
查询工资不是3000、3500、4000、9000的员工姓名和工资。
mysql> SELECT name,salary FROM employee WHERE salary NOT IN (3000,3500,4000,9000) ; +-----------+------------+ | name | salary | +-----------+------------+ | egon | 7300.33 | | alex | 1000000.31 | | wupeiqi | 8300.00 | | liwenzhou | 2100.00 | | jinxin | 30000.00 | | xiaomage | 10000.00 | | 歪歪 | 3000.13 | | 丫丫 | 2000.35 | | 丁丁 | 1000.37 | | 星星 | 3000.29 | | 格格 | 4000.33 | | 张野 | 10000.13 | | 程咬金 | 20000.00 | | 程咬银 | 19000.00 | | 程咬铜 | 18000.00 | | 程咬铁 | 17000.00 | +-----------+------------+ 16 rows in set (0.00 sec)
（5）关键字like模糊查询
查找名字以**开头的员工所有信息。
通配符’%’ mysql> SELECT * FROM employee WHERE name LIKE ‘jin%‘; +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ | id | 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 | +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+ 2 rows in set (0.00 sec)
通配符‘_‘ mysql> SELECT age FROM employee WHERE name LIKE ‘ale_‘; +-----+ | age | +-----+ | 78 | +-----+ 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30岁的员工姓名、年龄 3. 查看岗位是teacher且薪资在9000-1000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jin开头的员工姓名、年薪
答案：
select name,age from employee where post = ‘teacher‘; select name,age from employee where post=‘teacher‘ and age > 30; select name,age,salary from employee where post=‘teacher‘ and salary between 9000 and 10000; select * from employee where post_comment is not null; select name,age,salary from employee where post=‘teacher‘ and salary in (10000,9000,30000); select name,age,salary from employee where post=‘teacher‘ and salary not in (10000,9000,30000); select name,salary*12 from employee where post=‘teacher‘ and name like ‘jin%‘;
2、group by分组查询
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，
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。
假如要分组查询工资的员工名字，直接select name,salary from employee group by salary;只能查询出每中工资第一个员工的信息，没有任何意义。
这个时候
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