MySql的用法总结-2

一、 单表查询

1、where 条件的使用

　　功能:对表中的数据进行筛选和过滤

　　语法:
　　　　1.判断的符号
　　　　　　> < >= <= = !=( <>不等于 )
　　　　2.拼接不同条件的关键字
　　　　　　and or not
　　　　3.查询区间值
　　　　　　between 小值 and 大值 [小值,大值] 查询两者之间的范围
　　　　4.查询区间值
　　　　　　id in (1,2,3,4,5,6)
　　　　5.模糊查询 like %通配符 _通配符
　　　　　　like "%b" 匹配以b结尾的任意长度字符串
　　　　　　like "a%" 匹配以a开头的任意长度字符串
　　　　　　like "%c%" 匹配字符串中含有c的任意长度字符串
　　　　　　like "__d" 匹配总长度为3位,而且以d结尾的字符串
　

　　　　　like "e__" 匹配总长度为3位,而且以e开头的字符

 #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
);

#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)
;

# 1. 查询部门是sale的所有员工姓名:
select emp_name from employee where post="sale"

# 2. 部门是teacher , 收入大于10000的所有数据
select * from employee where post="teacher" and salary > 10000;

# 3. 收入在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary between 10000 and 20000;

# 4. 收入不在1万到2万之间的所有员工姓名和收入
select emp_name,salary from employee where salary not between 10000 and 20000;

# 5. 查看岗位描述为NULL的员工信息
select * from employee where post_comment is null;
update employee set post_comment = "" where id = 1;
select * from employee where post_comment = "";
# 查看岗位描述不为NULL的员工信息 
select * from employee where  post_comment is not null;

# 6. 查询收入是3000 ,4000 ,5000,8300 所有员工的姓名和收入
select emp_name,salary from employee where salary in(3000,4000,5000,8300); # (推荐)
select emp_name,salary from employee where salary = 3000 or salary = 4000 or salary = 5000 or salary = 8300;

# 查询收入不是3000 ,4000 ,5000,8300 所有员工的姓名和收入
select emp_name,salary from employee where salary not in(3000,4000,5000,8300); # (推荐)

# 7. 以on结尾的员工名搜一下
select emp_name from employee where emp_name like "%on";
select emp_name from employee where emp_name like "wu%";
select emp_name from employee where emp_name like "%le%";
select emp_name from employee where emp_name like "al__";
select emp_name from employee where emp_name like "%alex%";

# 8. 统计员工一年的年薪
select concat("姓名:",emp_name,"收入:",salary)  from employee;
# + - * / 四则运算
select concat("姓名:",emp_name,"收入:",salary * 12 )  from employee;
# 语法: concat_ws(拼接符号,字段1,字段2,字段3 .... )
select concat_ws(" : ",emp_name,salary * 12 )  from employee;

# 9. 查询部门的种类
# distinct 去重
select distinct(post) from  employee

 

2、 group by 子句  分类,分组

注意点: 针对于当前表,by谁搜谁

select sex from employee group by sex
select emp_name from employee group by sex # error
# group_concat 按照分组把对应的字段拼接在一起
select group_concat(emp_name) from employee group by sex;

# 聚合函数
    # count 统计数量 * 号代表所有 
    select count(*) from employee
    # max 统计最大值
    select max(salary) from employee;
    # min 统计最大值
    select min(salary) from employee;
    # avg 统计平均值
    select avg(salary) from employee;
    # sum 统计总和
    select sum(salary) from employee;

#练习:group 分类
# 1. 查询部门名以及各部门的平均薪资
select avg(salary),post from employee  group by post

# 2. 查询部门名以及各部门的最高薪资
select max(salary),post from employee  group by post

# 3. 查询部门名以及各部门的最低薪资
select min(salary),post from employee  group by post

# 4. 查询公司内男员工和女员工的个数
select count(*),sex from employee  group by sex

# 5. 查询部门名以及部门包含的所有员工名字
select group_concat(emp_name),post from employee group by post;

# 6. 可以group by 两个字段,by谁搜谁;
select emp_name,post from employee group by post,emp_name;    

3、.having 对分类后的数据进行二次过滤[应用在group by这个场景里]

# 找出各部门平均薪资,且大于10000
select post,avg(salary) from  employee group by post having  avg(salary) > 10000

# 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 group_concat(emp_name),post,count(*) from employee group by post having count(*) > 2

# 2.查询各岗位平均薪资小于10000的岗位名、平均工资
select post,avg(salary) from  employee group by post having  avg(salary) < 10000

# 3.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  10000 < avg(salary)< 20000  error[没有搜到想要的结果]
select post,avg(salary) from  employee group by post having  10000 < avg(salary) and   avg(salary)  < 20000
# 10000 <= avg(salary) <= 20000
select post,avg(salary) from  employee group by post having  avg(salary) between 10000 and 20000;

4、.order by 排序

• 正序 升序 asc
• 倒序 降序 desc

#练习:order by
select * from employee order by age;
select * from employee order by age asc;(默认升序)
select * from employee order by age desc;(默认升序)

# 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 asc , 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;

5、.limit 限制查询的条数

 　　limit m,n  m代表从第几条搜索数据 , n 代表搜索几条 m=0 代表搜索第一条数据 

# 分页
select * from employee limit 0,10  # 0代表第一条 ,往后搜10条数据
select * from employee limit 10,10 # 10代表第11条,往后搜10条数据
select * from employee limit 20,10 # 20代表第21条,往后搜10条数据

# limit 数字  代表搜索条数
select * from employee limit 1;
# 搜索表里面最后一条数据
select * from employee order by id desc limit 1;
# 搜索表里面最后三条数据
select * from employee order by id desc limit 3;

6、(了解) 可以使用正则表达式 (不推荐使用)

select * from employee where emp_name regexp ".*n$"; #?号不识别
select * from employee where emp_name regexp "程咬.*"; 

二、多表查询

 #建表
 create table department(
 id int,
 name varchar(20) 
 );
  
 create table employee(
 id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);
 
#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

1、内联查询(内联接)

　　inner join  至少两表以上做查询,把满足条件的所有数据查询出来(查询的是共同拥有的数据)

• select 字段 from 表1 inner join 表2 on 必要的关联字段 (2张表)
• select 字段 from 表1 inner join 表2 on 必要的关联字段1 inner join 表3 on 必要的关联字段2 ... inner join ...

# 语法:
select * from employee inner join  department on employee.dep_id = department.id ;
# as 起别名 (推荐)
select * from employee as e inner join  department as d on e.dep_id = d.id ;
# as 可以省略
select * from employee  e inner join  department  d on e.dep_id = d.id ;

# where 写法默认等价于inner join 也是内联查询
select * from employee , department  where  employee.dep_id = department.id ;
select * from employee as e, department as d  where  e.dep_id = d.id ;

2、外联查询(外联接)

# (1) left join (左联接) : 以左表为主,右表为辅,完整查询左表所有数据,右表没有的数据补null 
select * from employee left join   department on employee.dep_id = department.id ;
# (2) right join (右联接): 以右表为主,左表为辅,完整查询右表所有数据,左表没有的数据补null 
select * from employee right join   department on employee.dep_id = department.id ;

3、全联查询(全联接) left join + right join 

select * from employee left join   department on employee.dep_id = department.id 
union
select * from employee right join   department on employee.dep_id = department.id ;

三、子查询

子查询 : sql语句的嵌套
(1) sql语句当中嵌套另外一条sql,用括号()包起来,表达一个整体;
(2) 一般用在子句的后面 比如from , where ...身后 表达一个条件或者一张表
(3) 速度快慢 : 单表查询 > 联表查询 > 子查询

#建表
create table department(
id int,
name varchar(20) 
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

#插入数据
insert into department values
(200,'技术'),
(201,'人力资源'),
(202,'销售'),
(203,'运营');

insert into employee(name,sex,age,dep_id) values
('egon','male',18,200),
('alex','female',48,201),
('wupeiqi','male',38,201),
('yuanhao','female',28,202),
('liwenzhou','male',18,200),
('jingliyang','female',18,204)
;

1、基本用法

1、找出平均年龄大于25岁以上的部门

# where 
select 
    department.id,department.name
from
    employee,department
where 
    employee.dep_id = department.id
group by
    department.id,department.name
having 
    avg(age) > 25;

# 用as 起别名
select 
    d.id,d.name
from
    employee as e,department as d
where  
    e.dep_id = d.id
group by
    d.id,d.name
having 
    avg(age) > 25;

# inner join 
select 
    d.id,d.name
from
    employee as e inner join  department as d on e.dep_id = d.id
group by
    d.id,d.name
having 
    avg(age) > 25;

2、子查询练习

1.找平均年龄大于25岁以上的部门id

select 
    dep_id
from 
    employee 
group by 
    employee.dep_id
having 
    avg(age) > 25
# 2.通过id上 department 表里面找部门名
select name from department where id in(201,202);

# 3.综合拼接
select id,name from department where id in(select  dep_id from employee group by  employee.dep_id having avg(age) > 25);

2.查看技术部门员工姓名

# where 
select 
    e.name
from 
    employee as e , department as d
where
    e.dep_id = d.id
    and
    d.name = "技术"
    
# inner join 
select 
    e.name
from 
    employee as e inner join  department as d on e.dep_id = d.id
where 
    d.name = "技术"


# 子查询
# (1) 通过技术部门找id
select id from department where name = "技术"
# (2) 通过id 找员工姓名
select name from employee where dep_id = 200;
# (3) 综合拼接
select name from employee where dep_id = (select id from department where name = "技术");

3.查看哪个部门没员工

# 联表查询
select 
    d.id,d.name 
from 
    department as d left join employee  as e on d.id = e.dep_id 
where 
    e.id is null

# 子查询
# 1.找员工都在哪些部门
select dep_id from employee group by dep_id

# 2把不在该部门的员工找出来
select id from department where id not in (200,201,202,204);

# 综合拼接
select id , name  from department where id not in (select dep_id from employee group by dep_id);

4.查询大于平均年龄的员工名与年龄

select name , age  from employee where age > 28
# 计算平均年龄
select  avg(age) from employee;
# 综合拼接
select name , age  from employee where age > (select  avg(age) from employee);

5.把大于其本部门平均年龄的员工名和姓名查出来

# 1.先计算本部门的平均年龄是多少
select dep_id,avg(age) from employee  group by dep_id
# 2.把搜索出来的数据和employee表进行联表,最后做单表查询
select 
    *
from
    employee as t1 inner join (1号sql查询出来的数据) as t2 on t1.dep_id = t2.dep_id

# 3.综合拼接
select 
    *
from
    employee as t1 inner join (select dep_id,avg(age) from employee  group by dep_id) as t2 on t1.dep_id = t2.dep_id
    
# 4.做单表查询
select 
    t1.name
from
    employee as t1 inner join (select dep_id,avg(age) as avg_age from employee  group by dep_id) as t2 on t1.dep_id = t2.dep_id
where 
    t1.age > t2.avg_age

6.查询每个部门最新入职的那位员工  # 利用上一套数据表进行查询;

# 1.找每个部门 hire_date 字段的最大值(即是最新入职的员工)
select max(hire_date) as max_date , post from employee  group by post
# 2.把 employee 和 子查询搜出的最大日期做联表,合并成大表之后,在做单表查询;
select 
    *
from 
    employee as t1 inner join (1号查询出来的数据) as t2 on t1.post = t2.post
    
# 3.综合拼接
select 
    *
from 
    employee as t1 inner join (select max(hire_date) as max_date , post from employee  group by post) as t2 on t1.post = t2.post
    
# 4.最后做单表查询
select 
    t1.emp_name,t1.hire_date
from 
    employee as t1 inner join (select max(hire_date) as max_date , post from employee  group by post) as t2 on t1.post = t2.post
where 
    t1.hire_date = t2.max_date

7 总结:

 子查询可以作为临时表,也可以作为where子句的条件,通过()包括sql,表达一个整体;
     一般用在各个子句后面 select .. from ... where ...
思路:
   可以把临时搜索出来的数据变成临时表,在和其他表做联表,最后做单表查询;

3.带EXISTS关键字的子查询

exists 关键字,表达数据是否存在,用在子查询里
如果内层sql 能够查到数据,返回True ,外层sql执行sql语句
如果内层sql 不能够查到数据,返回False ,外层sql不执行sql语句

select * from employee where exists (select * from employee where id = 1);
select * from employee where exists (select * from employee where id = 100);