MySQL简单查询和单表查询

mysql记录操作

概览

mysql数据操作： dml

在mysql管理软件中，可以通过sql语句中的dml语言来实现数据的操作，包括

1. 使用insert实现数据的插入
2. update实现数据的更新
3. 使用delete实现数据的删除
4. 使用select查询数据以及。

插入数据insert

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 …;

更新数据update

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’;

删除数据delete

语法：
    delete from 表名 
        where conition;

示例：
    delete from mysql.user 
        where password=’’;

练习：
    更新mysql root用户密码为mysql123
    删除除从本地登录的root用户以外的所有用户

查询数据 search

1.单表查询

关键字执行的优先级:

from
where
group by
select
distinct
having
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.执行select（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

简单查询

#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','办事处',7300.33,401,1), #以下是教学部
('anwen','male',78,'20150302','teacher',1000000.31,401,1),
('wudi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwen','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xx','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

准备表和记录

#简单查询
    select id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    from employee;

    select * from employee;

    select emp_name,salary from employee;

#避免重复distinct
    select distinct post from employee;    

#通过四则运算查询
    select emp_name, salary*12 from employee;
    select emp_name, salary*12 as annual_salary from employee;
    select emp_name, salary*12 annual_salary from employee;

#定义显示格式
   concat() 函数用于连接字符串
   select concat('姓名: ',emp_name,'  年薪: ', salary*12)  as annual_salary 
   from employee;
   
   concat_ws() 第一个参数为分隔符
   select concat_ws(':',emp_name,salary*12)  as annual_salary 
   from employee;

   结合case语句：
   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;

小练习：

1 查出所有员工的名字，薪资,格式为
    <名字:egon>    <薪资:3000>
2 查出所有的岗位（去掉重复）
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 concat('<名字:',emp_name,'>    ','<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select emp_name,salary*12 annual_salary from employee;

where约束

where字句中可以使用：

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80或90或100
4. like 'e%'
       通配符可以是%或_，
       %表示任意多字符
       _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
    select emp_name from employee
        where post='sale';
        
#2:多条件查询
    select emp_name,salary from employee
        where post='teacher' and salary>10000;

#3:关键字between and
    select emp_name,salary from employee 
        where salary between 10000 and 20000;

    select emp_name,salary from employee 
        where salary not between 10000 and 20000;
    
#4:关键字is null(判断某个字段是否为null不能用等号，需要用is)
    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;
        
    select emp_name,post_comment from employee 
        where post_comment=''; 注意''是空字符串，不是null
    ps：
        执行
        update employee set post_comment='' where id=2;
        再用上条查看，就会有结果了

#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’%’
    select * from employee 
            where emp_name like 'eg%';

    通配符’_’
    select * from employee 
            where emp_name like 'al__';

小练习：

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

select emp_name,age from employee where post = 'teacher';
select emp_name,age from employee where post='teacher' and age > 30; 
select emp_name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select emp_name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select emp_name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select emp_name,salary*12 from employee where post='teacher' and emp_name like 'jin%';

group by

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
    select post,group_concat(emp_name) from employee group by post;#按照岗位分组，并查看组内成员名
    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
    select post,count(id) as count from employee group by post;#按照岗位分组，并查看每个组有多少人


强调：
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

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

示例：
    select count(*) from employee;
    select count(*) from employee where depart_id=1;
    select max(salary) from employee;
    select min(salary) from employee;
    select avg(salary) from employee;
    select sum(salary) from employee;
    select sum(salary) from employee where depart_id=3;

小练习：

1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
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

#题1：分组
mysql> select post,group_concat(emp_name) from employee group by post;
#题目2：
mysql> select post,count(id) from employee group by post;
#题目3：
mysql> select sex,count(id) from employee group by sex;
#题目4：
mysql> select post,avg(salary) from employee group by post;
#题目5
mysql> select post,max(salary) from employee group by post;
#题目6
mysql> select post,min(salary) from employee group by post;
#题目七
mysql> select sex,avg(salary) from employee group by sex;

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

select * from emp where salary > 100000;

mysql> select post,group_concat(emp_name) from emp 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 'salary' in 'having clause'

mysql> select post,group_concat(emp_name) from emp group by post having avg(salary) > 10000;

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
3. 查询各岗位平均薪资大于10000的岗位名、平均工资
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,group_concat(emp_name),count(id) from employee group by post having count(id) < 2;
select post,avg(salary) from employee group by post having avg(salary) > 10000;
select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;

order by 查询排序

按单列排序
    select * from employee order by salary;
    select * from employee order by salary asc;
    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 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

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

limit 限制查询的记录数

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：

select * from  employee limit 0,5;
select * from  employee limit 5,5;
select * from  employee limit 10,5;

使用正则表达式查询

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

select * from employee where emp_name regexp 'on$';

select * from employee where emp_name regexp 'm{2}';


小结：对字符串匹配的方式
where emp_name = 'egon';
where emp_name like 'yua%';
where emp_name regexp 'on$';