day39:MySQL:查询操作之单表查询&多表查询&子查询

#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

);

#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;