mysql 单表查询

目录

• • having过滤

单表查询

准备数据

• 只是把查询结果按照自己想要的方式返回,不对数据做修改

#准备表和记录
company.employee  #设计表结构
    员工id      id                  int             
    姓名        emp_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

#建表和数据准备
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    |                |
+--------------+-----------------------+------+-----+---------+----------------+

#插入数据
#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','鸡汤导师',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)
;

#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

简单查询

• 指定字段查询

• select id ,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id from employee;

• select emp_name,salary from employee;

• 查看所有字段

• select * 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, '棒棒哒') end ) as new_name from employee;

比较运算符

• 大于 > 小于< 等于= 大于等于 >= 小于等于<= 不等于 != 不等于<>

范围筛选 between

• between 80 and 100 值在80到100之间

• 在一个模糊的范围里 between 10000 and 20000 salary 是1w-2w之间的所有人的名字

• select emp_name from employee where salary between 10000 and 20000;

多选 in

• 字段名 in (值1,值2，值3)
• select * from employee where salary in (20000,30000,3000,19000,18000,17000);

模糊查询 like

• 通配符可以是%或**_** %表示任意多字符 _表示一个字符

• select * from employee where emp_name like '程%';

​

逻辑运算符：在多个条件直接可以使用逻辑运算符 and or not

• 逻辑运算 - 条件的拼 (与 and) (或 or) (非 not)

• select * from employee where emp_name not in ('程咬金','程咬银','程咬铜');

• select * from employee where emp_name not like ('程%');

• select emp_name,salary*12 from employee where post='operation' and emp_name like '程%';

  • 查看岗位是operation且名字是程开头的员工姓名、年薪
    

• select * from employee where salary = 20000 or salary = 30000;

• select * from employee where emp_name = '丫丫' or emp_name = '丁丁' or emp_name = '星星' or emp_name = '格格'or salary = 20000;

身份运算 - 关于null is null /is not null

• 查看岗位描述为null的员工信息

• select * from employee where post_comment is not null;

正则匹配 regexp

• 更加细致的匹配的时候使用 select * from 表 where 字段 regexp 正则表达式

• select emp_name,salary*12 from employee where post='teacher' and emp_name regexp '^jin.*'

• select * from employee where emp_name regexp '^j[a-z]{5}'

分组 group by

#单独使用group by关键字分组
    mysql> select post from employee group by post;
    +--------------+
    | post         |
    +--------------+
    | operation    |
    | sale         |
    | teacher      |
    | 鸡汤导师     |
    +--------------+
    4 rows in set (0.02 sec)
    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
    mysql> 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
    +--------------+---------------------------------------------------------+
    | post         | group_concat(emp_name)                                  |
    +--------------+---------------------------------------------------------+
    | operation    | 程咬铁,程咬铜,程咬银,程咬金,张野                        |
    | sale         | 格格,星星,丁丁,丫丫,歪歪                                |
    | teacher      | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex   |
    | 鸡汤导师     | egon                                                    |
    +--------------+---------------------------------------------------------+
    4 rows in set (0.04 sec)

    mysql> select post,group_concat(emp_name) as emp_members from employee group by post;
    +--------------+---------------------------------------------------------+
    | post         | emp_members                                             |
    +--------------+---------------------------------------------------------+
    | operation    | 程咬铁,程咬铜,程咬银,程咬金,张野                        |
    | sale         | 格格,星星,丁丁,丫丫,歪歪                                |
    | teacher      | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex   |
    | 鸡汤导师     | egon                                                    |
    +--------------+---------------------------------------------------------+
    4 rows in set (0.00 sec)

#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
    +--------------+-------+
    | post         | count |
    +--------------+-------+
    | operation    |     5 |
    | sale         |     5 |
    | teacher      |     7 |
    | 鸡汤导师     |     1 |
    +--------------+-------+
    4 rows in set (0.02 sec)

• select * from employee group by post;

• 分组会把在group by后面的这个字段，也就是post字段中的每一个不同的项都保留下来

• 并且把值是这一项的的所有,行归为一组,但是只显示第一个分组匹配上值的所有数据,且一般默认倒序显示,数值的默认从小到大排序.

聚合 (把很多行的同一个字段进行一些统计,最终的到一个结果)

#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：
# count(字段/*) 统计这个字段有多少项
    select count(id) from employee;
    +-----------+
    | count(id) |
    +-----------+
    |        18 |#结果
    +-----------+
    1 row in set (0.00 sec)
    
    select count(*) from employee where depart_id=1;
    +----------+
    | count(*) |
    +----------+
    |        8 |#结果
    +----------+
    1 row in set (0.00 sec)
    
    
# max(字段)    统计这个字段对应的数值的最大值(薪资)   
    select max(salary) from employee;
    +-------------+
    | max(salary) |
    +-------------+
    |  1000000.31 |#结果
    +-------------+
    1 row in set (0.03 sec)
    
# min(字段)    统计这个字段对应的数值的最小值(薪资)    
    select min(salary) from employee;
    +-------------+
    | min(salary) |
    +-------------+
    |     1000.37 |#结果
    +-------------+
    1 row in set (0.00 sec)

    
# avg(字段)   统计这个字段对应的数值的平均值(薪资)   
    select avg(salary) from employee;
    +--------------+
    | avg(salary)  |
    +--------------+
    | 64844.568889 |#结果
    +--------------+
    1 row in set (0.00 sec)
    
# sum(字段)   统计这个字段对应的数值的和
    mysql> select sum(salary) from employee;
    +-------------+
    | sum(salary) |
    +-------------+
    |  1167202.24 |#结果
    +-------------+
    1 row in set (0.00 sec)
    
    select sum(salary) from employee where depart_id=3;
    +-------------+
    | sum(salary) |
    +-------------+
    |    84000.13 |#结果
    +-------------+
    1 row in set (0.00 sec)

分组+聚合

一 concat()函数
1、功能：将多个字符串连接成一个字符串。
2、语法：concat(str1, str2,...)　　
返回结果为连接参数产生的字符串，如果有任何一个参数为null，则返回值为null。
3、语法：concat(str1, seperator,str2,seperator,...)
返回结果为连接参数产生的字符串并且有分隔符，如果有任何一个参数为null，则返回值为null。

二 concat_ws()函数
1、功能：和concat()一样，将多个字符串连接成一个字符串，但是可以一次性指定分隔符（concat_ws就是concat with separator）

2、语法：concat_ws(separator, str1, str2, ...)

说明：第一个参数指定分隔符。需要注意的是分隔符不能为null，如果为null，则返回结果为null。
 
三 group_concat()函数
1、功能：将group by产生的同一个分组中的值连接起来，返回一个字符串结果。
2、语法：group_concat( [distinct] 要连接的字段 [order by 排序字段 asc/desc  ] [separator '分隔符'] )
说明：通过使用distinct可以排除重复值；如果希望对结果中的值进行排序，可以使用order by子句；separator是一个字符串值，缺省为一个逗号。

1. 查询岗位名以及岗位包含的所有员工名字
mysql> select post,group_concat(emp_name) from employee group by post;
+--------------+---------------------------------------------------------+
| post         | group_concat(emp_name)                                  |
+--------------+---------------------------------------------------------+
| operation    | 程咬铁,程咬铜,程咬银,程咬金,张野                          |
| sale         | 格格,星星,丁丁,丫丫,歪歪                                 |
| teacher      | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex   |
| 鸡汤导师      | egon                                                    |
+--------------+---------------------------------------------------------+


2. 查询岗位名以及各岗位内包含的员工个数
mysql> select post,count(emp_name) from employee group by post ;
+--------------+-----------------+
| post         | count(emp_name) |
+--------------+-----------------+
| operation    |               5 |
| sale         |               5 |
| teacher      |               7 |
| 鸡汤导师     |               1 |
+--------------+-----------------+


3. 查询公司内男员工和女员工的个数
select sex, count(id) from employee group by sex;
+--------+-----------+
| sex    | count(id) |
+--------+-----------+
| male   |        10 |
| female |         8 |
+--------+-----------+


4. 查询岗位名以及各岗位的平均薪资
select post,avg(salary) from employee group by post;
+--------------+---------------+
| post         | avg(salary)   |
+--------------+---------------+
| operation    |  16800.026000 |
| sale         |   2600.294000 |
| teacher      | 151842.901429 |
| 鸡汤导师     |   7300.330000 |
+--------------+---------------+


5. 查询岗位名以及各岗位的最高薪资
select post,max(salary) from employee group by post;
+--------------+-------------+
| post         | max(salary) |
+--------------+-------------+
| operation    |    20000.00 |
| sale         |     4000.33 |
| teacher      |  1000000.31 |
| 鸡汤导师     |     7300.33 |
+--------------+-------------+


6. 查询岗位名以及各岗位的最低薪资
select post,min(salary) from employee group by post;
+--------------+-------------+
| post         | min(salary) |
+--------------+-------------+
| operation    |    10000.13 |
| sale         |     1000.37 |
| teacher      |     2100.00 |
| 鸡汤导师     |     7300.33 |
+--------------+-------------+


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
select sex,avg(salary) from employee group by sex;
+--------+---------------+
| sex    | avg(salary)   |
+--------+---------------+
| male   | 110920.077000 |
| female |   7250.183750 |
+--------+---------------+

having过滤

#having与where不一样的地方在于!!!!!!
1. 执行优先级从高到低：where > group by > having 
2. 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。
3. 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

• having 条件 # 过滤 组

• select count(id) as '人数',post from employee group by post having count(*) > 3; 找出部门人数大于3的部门

• select post '均薪资>10000部门' from employee group by post having avg(salay)>10000; 平均薪资大于10000的部门

• select * from employee having age>18 年龄大于18岁的

• order by 查询排序

  • order by 某一个字段 asc; 默认是升序asc(ascending升序) 从小到大

  • select emp_name,age from employee order by age; 年龄按照从小到大排序

• order by 某一个字段 desc; 指定降序排列desc(descending)从大到小

• select emp_name,age from employee order by age desc; 年龄按照从大到小排序

• select emp_name,hire_date from employee order by hire_date desc; 入职日期从早到晚排序

• order by 第一个字段 asc,第二个字段 desc;指定先根据第一个字段升序排列，在第一个字段相同的情况下，再根据第二个字段排列

• select emp_name,age,salary from employee order by age,salary desc;

• limit 限制查询的记录数

  • 取前n个 limit n == limit 0,n

  • select emp_name,salary from employee order by salary desc limit 3; 找出薪资最高的三个人,从多到少排序

• 分页 limit m,n 从m+1开始取n个

• 员工展示的网页,18个员工,每一页展示5个员工

• limit n offset m == limit m,n 从m+1开始取n个

  • select * from employee order by id desc limit 0,5;
    
  • select * from employee order by id desc limit 5,5;
  • select * from employee order by id desc limit 10,5;
  • select * from employee order by id desc limit 15,5;