day39:MySQL:查询操作之单表查询&多表查询&子查询

目录

part1:单表查询

　　1.where条件的使用

　　2.group 子句 分组分类

　　3.having 数据在分类分组之后,进行二次数据过滤

　　4.order by 排序, 按照什么字段进行排序

　　5.limit 限制查询条数 (数据分页)

　　6.regexp (了解)可以使用正则表达式查询数据 (不推荐,效率不高)

part2:多表查询

　　1.内连接 inner join

　　2.外连接

part3:子查询

part1:单表查询

SQL查询语句的完整语法: select .. from .. where .. group by .. having  .. order by .. limit ..

1.where条件的使用

功能: 对表中的数据进行过滤筛选

1.判断的符号

　　= > >= < <= != <> 不等于

2.拼接关键字

　　and or not

3.查询范围区间 between

　　between 小值 and 大值 [小值,大值] 查询两者之间的这个范围所有数据

4.查询某个值在具体某个范围里 in

　　in(1,2,3,4)

5.模糊查询 like "%" "_" 通配符

　　like "%a" 匹配以a结尾的任意长度的字符串

　　like "a%" 匹配以a开头的任意长度的字符串

　　like "%a%" 匹配含有a字母的任意长度的字符串

　　like "_a" 个数一共是2个字符,必须以a结尾,前面字符随意

　　like "a__" 个数一共是3个字符,必须以a开头,后面字符随意

创建employee并插入数据

#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) 单条件的查询
    # 查询部门是sale 的所有员工姓名
    select emp_name from employee where post = "sale";
    
    # (2) 多条件的查询
    # 部门是teacher , 收入大于10000的所有数据
    select * from employee where post="teacher" and salary > 10000;
    
    # (3) between .. and ..
    # 收入在1万~2万之间的所有员工姓名和收入
    select emp_name,salary from employee where salary between 10000 and 20000;
    # 收入不在1万~2万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;
    
    # (4) null 关键字 在查询时候,需要使用is进行判断 ,不能用=
    select * from employee where post_comment = null;
    select * from employee where post_comment = '';
    select * from employee where post_comment is null;
    select * from employee where post_comment is not null;
    
    # (5) in  在  ... 之中
    # 查询收入是 3000 ,4000,5000 ,8300所有原型的姓名和收入
    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;
    # not .. in ..
    select emp_name,salary from employee where salary not in (3000,4000,5000,8300);
    
    # (6) 模糊查询 like "%" "_"
    # (1) 匹配员工姓名 以on结尾的.  "%" 通配符 
    select emp_name from employee where emp_name like "%on";
    # (2) "_"通配符 限定字符长度使用_
    select emp_name from employee where emp_name like "a_e_";
    
    # (7) concat  (as 起别名)
    select  concat("姓名:",emp_name,"工资:",salary) as ceshi  from employee;
    # concat_ws(拼接的符号, 参数1,参数2,参数3 .. .. )
    select  concat_ws(" : " , emp_name , salary) as ceshi  from employee;
    # 计算每个人的年薪 可以在mysql使用四则运算(+ - * / )
    select  concat_ws(" : " , emp_name,  salary * 12) as ceshi200 from employee;

employee表详情如下所示

2.group 子句 分组分类

group by 字段 对当前字段进行分类 , by后面接什么字段,select 就搜什么字段

    """group by 字段 对当前字段进行分类 , by后面接什么字段,select 就搜什么字段"""
    select sex from employee group by sex;
    select post from employee group by post;
    # group_concat 按照分类的形式进行字段的拼接
    select group_concat(emp_name) from employee group by post;

    # 聚合函数
        # count 统计总数  *所有
        select count(*) from employee;
        # max   统计最大值
        select max(salary) from employee;
        # min   统计最小值
        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;

    
    # 1. 查询部门名以及各部门的平均薪资 聚合函数 + 分组  配合使用
    select post,avg(salary) from employee  group by post
    
    # 2. 查询部门名以及各部门的最高薪资
    select post,max(salary) from employee  group by post

    # 3. 查询部门名以及各部门的最低薪资
    select post,min(salary) from employee  group by post
    
    # 4. 查询公司内男员工和女员工的个数
    select sex, count(*) 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 post,group_concat(emp_name)  from employee group by post
    # 可以group by 两个字段,搜索2个字段
    select post , emp_name from employee group by post , emp_name

3.having 数据在分类分组之后,进行二次数据过滤,一般是配合group by 使用,分组之后过滤

    # 找平均薪资大于10000以上的所有部门
    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 post , group_concat(emp_name) , 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 avg(salary) > 10000 and avg(salary) < 20000;

4.order by 排序, 按照什么字段进行排序

asc 升序: 从小到大 (默认)

desc 降序: 从大到小

    select * from employee order by age #(默认是asc升序)
    select * from employee order by age desc #(默认是desc 倒序)

    # 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  , 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 限制查询条数 (数据分页)

limit m,n m代表从第几条数据进行查询,0代表第一条,n代表的查询几条

    select * from employee limit 0,5  # 从第一条数据开始搜, 搜5条数据
    select * from employee limit 5,5  # 从第六条数据开始搜, 搜5条数据
    
    # 只搜索一条数据
    select * from employee limit 1
    # 只搜索3条数据
    select * from employee limit 3
    # 搜索这个表中最后一条数据
    select * from employee order by id desc limit 1

6.regexp (了解)可以使用正则表达式查询数据 (不推荐,效率不高)

    select * from employee where emp_name regexp ".*on$"; # .*? 这个?号 mysql 不识别
    select * from employee where emp_name regexp "程";
    select * from employee where emp_name regexp "程.*金";

part2:多表查询

创建employee表和department表，并插入数据

#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)
;

dapartment表和employee表详情如图所示

1.内连接 inner join

内连接 : (inner join ) -> 两表或者多表满足条件的所有数据查询出来(两表之间的共有数据)

两表查询

　　select 字段 from 表1 inner join 表2 on 必要的关联条件

多表查询

　　select 字段 from 表1 inner join 表2 on 必要的关联条件1 inner join 表3 on 必要的关联条件2 ...

    # 基本语法 inner join on + 条件
    select * from employee inner join department on employee.dep_id = department.id;
    # 用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 默认实现的就是内联查询的效果
    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.外连接

1.左连接(左联查询 left join )  以左表为主,右表为辅,完整查询左表所有数据,右表没有的补null

2.右连接(右联查询 right join ) 以右表为主,左表为辅,完整查询右表所有数据,左表没有的补null

3.全连接 union

        # 1.左连接(左联查询 left join )  以左表为主,右表为辅,完整查询左表所有数据,右表没有的补null
        select * from employee left join department on employee.dep_id = department.id;
        # 2.右连接(右联查询 right join ) 以右表为主,左表为辅,完整查询右表所有数据,左表没有的补null
        select * from employee right join department on employee.dep_id = department.id;
        # 3.全连接
        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

part3:子查询

子查询: 嵌套查询

1.sql语句当中又嵌套了另外一条sql语句,用()括号抱起来,表达一个整体

2.一般应用在from 字符后面(表达一张表),where 子句后面(表达一个条件)

3.查询速度从快到慢 : 单表查询 -> 联表速度 -> 子查询

1.找出平均年龄大于25岁以上的部门

# (1) 普通where
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(e.age) > 25;
    
# (2) 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(e.age) > 25;
    
# (3)子查询
# 1.先选出平均年龄大于25岁的部门id
select dep_id from employee group by dep_id having avg(age) > 25; # 201 202
# 2.通过部门id,找部门的名字
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 dep_id having avg(age) > 25);

2.查看技术部门员工姓名

# (1) 普通where
select 
    e.name,d.name
from    
    employee as e , department as d
where 
    e.dep_id = d.id
    and
    d.name = "技术"

# (2) inner join 写法
select 
    e.name,d.name
from    
    employee as e inner join department as d  on e.dep_id = d.id
where     
    # 非必要条件写在where字句中
    d.name = "技术"

# (3) 子查询
# 1.找计数部门对应的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,dep_id from employee where dep_id = (select id from department where name ="技术");

3.查看哪个部门没员工

# 联表差生null值,谁是null谁就没员工
select 
    d.id,d.name
from 
    department as d left join  employee as e on e.dep_id = d.id
where
    e.id is null


# 1.查询员工都在哪些部门 (200 , 201 202 204)
select dep_id from employee group by dep_id 

# 2.把不在这些部门的数据找出来
select id from department where id not in (200,201,202,204)

# 3.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

# 如果平均年龄是25;
select name,age from employee where age > 25
# 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.先计算各部门平均年龄是多少
select dep_id,avg(age) as avg_age from employee group by dep_id

# 2.把查询各部门的平均年龄和过去employee做联表,变成更大的表方便后期做单表查询;
select 
*
from 
    employee as t1 inner join (1号数据) 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) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id

# 4.做最后的条件帅选
select 
*
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.找每个部门最大的入职时间
    select post , max(hire_date) as max_date from employee group by post;
    # 2.把子查询搜索出来的数据和 employee 联合成一张更大的表,做一次单表查询
    select 
        *
    from 
        employee as t1 inner join  (1号查询出来的数据) as t2 on t1.post = t2.post
    where
        t1.hire_date = t2.max_date

    # 综合拼接
　　select 
    　　t1.emp_name,t1.hire_date
　　from 
    　　employee as t1 inner join  (select post , max(hire_date) as max_date from employee group by post) as t2 on t1.post = t2.post
　　where
    　　t1.hire_date = t2.max_date

7.带EXISTS关键字的子查询

EXISTS 关键字,表达存在

如果内层sql 能够查询到数据,返回True ,外层sql执行查询语句

如果内层sql 不能够查询到数据,返回False ,外层sql不执行查询语句

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

8.关于子查询的总结

子查询总结:

子查询可以单独作为一个临时数据表,临时数据,临时字段

一般用在 from where select 子句后面

可以通过查询出来的临时数据和另外的表做一次联表,变成更大的表,然后做单表查询,以得到想要的结果.

9.distinct去重

    # 额外 distinct 去重[尝试操作]
    select distinct depart_id from employee;