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单表查询

时间:2019-10-22 09:56来源:MySQL数据库
本节重点: 主要内容: 1.查询语法 2.关键字的执行优先级 3.简单查询 4.单条件查询 5.where 约束 6.分组查询: group by 7.使用聚合函数查询 8.having 过滤 9.查询排序:order by 10限制查询的记录

本节重点:

主要内容:

1.查询语法

2.关键字的执行优先级

3.简单查询

4.单条件查询

5.where 约束

6.分组查询: group by

7.使用聚合函数查询

8.having 过滤

9.查询排序:order by

10限制查询的记录数:limit

11.使用正则表达式查询

先建数据库(文件夹)、后表(文件)带字段,后记录(内容)。

company.employee
    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



#创建表
create table employee(
id int not null unique auto_increment,
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 |
| 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(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)

  单表查询

一:查询语法:

select 字段1,字段2...  from  表名
             where  条件
             group by  field
             having   筛选
            order by field
            limit   限制条数

 

二:关键字的执行优先级(重点)

重点:关键字的优先级执行顺序
1.from
2.where
3.group by
4.having
5.select
6.distinct
7.order by
8.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.按照select后的字段得到一张新的虚拟表,如果有聚合函数,则将组内数据进行聚合

5.将4的结果过滤:having

6.查出结果:select

7.去重

8.将结果按条件排序:order by

9.限制结果的显示条数

语法:

三:简单查询

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

    select * from employee;

    select name,salary from employee;

#避免重复DISTINCT
    select distinct post from employee;    

#通过四则运算查询
    select name, salary*12 from employee;
    select name, salary*12 as Annual_salary from employee;
    select 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: ',name,'  年薪: ', salary*12)  AS Annual_salary 
   from employee;

   CONCAT_WS() 第一个参数为分隔符
   SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary 
   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

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select  concat('<名字:',name,’> ' , '<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select name,salary*12 annual_salary from employee;

View Code

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四:where  约束

强调: where是一种约束条件,mysql会拿着where指定的条件取表中取数据,而having则是在取出数据后进行过滤。

where字句中可以使用:

  1.比较运算符: >< >= <= <> !=

  2.between 80 and 100 值在 10 到 20 之间。

  3.in (80,90,100)值是10 或 20 或 30.

  4.like 'aray%' 

     patterm 可以是 %或_,

   %表示任意字符, _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 name from employee where post = 'sale';

2.多条件查询
    select name,salary from employee where post='teacher' and salary>10000;

3.关键字查询:between and
    select name,salary from employee where salary between 10000 and 20000;
    select name,salary from employee where sal''ary not between 10000 and 20000;

4.关键字查询:is null(判断某个字段是否为null 不能用等号们需要用is)
    select name,post_comment from employee where post_comment is null;
    select name,post_comment from employee where post_comment is not null;
    select name,post_comment from employee where post_comment ='';  #注意:''是空字符串。
    ps:
        执行
        update employee set post_comment='' where  id=2;
        再用上条查看,就会有结果了。

5.关键字查询: in    
    select name,salary from employee where salary=3000 or salary=3500 or salary=4000;
    select name,salary from employee where salary  in (3000,3500,4000);
    select name,salary from employee where salary not in (3000,3500,4000);

6.关键字查询:like
    通配符 '%'
    select * from employee where name like 'ar%';

    通配符: '_'
    select * from employee where name like 'ar__';

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

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1.
select name,age from employee where post ='teacher';

2.
select name,age from employee where post='teacher' and age>30;
''
3.
select name,age,salary from employee where post='teacher' and salary between 9000 adn 10000;

4.
select * from employee where post_comment is not null;

5.
select name,age,salary from employee where post='teacher' and salary  in(10000,9000,30000);

6.
select name,age,salary from employee where post='teacher' and salary not in(10000,9000,30000);

7.
select name,salary from empolyee where post='teacher' and name like 'jin%';

View Code

一、单表查询的语法
   SELECT 字段1,字段2... FROM 表名
                  WHERE 条件
                  GROUP BY field
                  HAVING 筛选
                  ORDER BY field
                  LIMIT 限制条数
二、关键字的执行优先级(重点)

重点中的重点:关键字的执行优先级
from
where
group by
having
select
distinct
order by
limit

五:分组查询: group by

大前提: 可以按照任意字段分组, 但分完组后,只能查看分组的那个字段,要想取的组内的其他字段信息,需要借助函数。

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,要获取组内的其他信息,要借助函数。

group by 关键字和 group_concat()函数一起使用。
    select post,group concat(name) from employee group by post;  #按岗位分组,并查看成员名。
    select post,group concat(name) as emp_members from employee group by post;

group ty 与聚合函数一起使用
    select post,count(id) as count from employee group by post;  #按岗位分组,并查看每个组有多少人

强调:

如果我们用unique 的字段做为 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。

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

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1.
select post,group_concat(name) from employee group by post;

2.
select post,count(id) from employee group by post;

3.
select sex,count(id) from employee group by sex;

4.
select post,avg(salary) from employee group by post;

5.
select post,max(salary) from employee group by post;

6.
select post,min(salary) from employee group by post;

7.
select sex,avg(salary) from employee group by sex;

View Code

1.找到表:from

六:使用聚合函数查询

先from找到表

再用where的条件约束去表中取出记录

然后进行分组group by,没有分组则默认一组

然后进行聚合

最后select出结果

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

2.拿着where指定的约束条件,去文件/表中取出一条条记录

七:having 过滤

having 与 where 在语法上是一样的。

select * from employee where salary > 10000;
select * from employee having salary > 10000;

不同点:

1.执行优先级:从高到低。
    where > group by > 聚合函数 > having 

2. where 是一个约束条件,是在结果返回之前起作用的。(先找到表,按where的约束条件,取出数据)
    having 是一个过滤条件,是在结果返回以后起作用的。(取出数据后,在group by 分组,如果没有group by 则所有的记录为一组,在执行聚合函数,然后使用having 对聚合的结果进行过滤),在having中可以用聚合函数,where则不能。

3.having 可以放到 group by 之后,而 where 只能放到group by 之前。

4.在查询过程中聚合语句有:
    (sum,min,max,avg,count) 要比having 子句优先执行,
    而where 子句在查询过程中执行优先级高于聚合语句。

验证不同之处:

#验证之前再次强调:执行优先级从高到低:where > group by > 聚合函数 > having 
select count(id) from employee where salary > 10000; #正确,分析:where先执行,后执行聚合count(id),然后select出结果
select count(id) from employee having salary > 10000;#错误,分析:先执行聚合count(id),后执行having过滤,无法对id进行salary>10000的过滤

#以上两条sql的顺序是
1:找到表employee--->用where过滤---->没有分组则默认一组执行聚合count(id)--->select执行查看组内id数目
2:找到表employee--->没有分组则默认一组执行聚合count(id)---->having 基于上一步聚合的结果(此时只有count(id)字段了)进行salary>10000的过滤,很明显,根本无法获取到salary字段

其他需要注意的问题:

select post,group_concat(name) from employee group by post having salary > 10000;#错误,分组后无法直接取到salary字段
select post,group_concat(name) from employee 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

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1.
select post,group_concat(name),count(id) from empoloyee group by post having count(id) < 2;

2.
select post,avg(salary)  from employee group by post having avg(salary) > 10000;

3.
select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) < 20000;

View Code

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

八:查询排序: order by

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

按多列排序:先按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

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1.
select * from employee order by age asc,hire_data desc;

2.
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;

3.
select post,avg(salary) from employee group by post having avg(salary) >10000
order bu avg(salary) desc;

View Code

4.将分组的结果进行having过滤

九: 限制查询的记录数: limit

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:

1. 分页显示,每页5条

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#题目1
mysql> select * from employee ORDER BY age asc,hire_date desc;

#题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+

#题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| teacher   | 151842.901429 |
| operation |  16800.026000 |
+-----------+---------------+

回到顶部
十 限制查询的记录数:LIMIT

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:
1. 分页显示,每页5条


mysql> select * from  employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name      | sex  | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
|  1 | egon      | male |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
|  2 | alex      | male |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  3 | wupeiqi   | male |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
|  4 | yuanhao   | male |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  5 | liwenzhou | male |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
5 rows in set (0.00 sec)

mysql> select * from  employee limit 5,5;
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
|  8 | 成龙       | male   |  48 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale    | NULL         |  2000.35 |    402 |         2 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec)

mysql> select * from  employee limit 10,5;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 11 | 丁丁      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |
| 12 | 星星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |
| 13 | 格格      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |
| 14 | 张野      | male   |  28 | 2016-03-11 | operation | NULL         | 10000.13 |    403 |         3 |
| 15 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec)

View Code

5.执行select

十:使用正则表达式查询

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

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

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

小结:对字符串匹配的方式
WHERE name = 'egon';
WHERE name LIKE 'yua%';
WHERE name REGEXP 'on$';

小练习:

查看所有员工中名字是jin开头,n或者g结果的员工信息

select * from employee where name regexp '^jin.*[gn]$';

  

 

6.去重

7.将结果按条件排序:order by

8.限制结果的显示条数

 

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创建公司员工表,表的字段和数据类型

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company.employee
    员工id          id                          int                  
    姓名            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

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#创建表,设置字段的约束条件
create table employee(
    id int primary key auto_increment,
    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
    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    |                |
| salart               | double(15,2)                  | YES  |         | NULL    |                |
| office              | int(11)                           | YES  |         | NULL    |                |
| depart_id        | int(11)                           | YES  |         | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+
10 rows in set (0.08 sec)

#插入记录
#三个部门:教学,销售,运营
insert into employee(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),
('xiaomage','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)
;

创建员工表,并插入记录

 

(1)where 约束 

凤凰彩票平台手机版 19

where子句中可以使用
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10或20或30
4.like 'xiaomagepattern': pattern可以是%或者_。%小时任意多字符,_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

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验证结果:

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#1 :单条件查询
mysql> select id,emp_name from employee where id > 5;
+----+------------+
| id | emp_name   |
+----+------------+
|  6 | jingliyang |
|  7 | jinxin     |
|  8 | xiaomage   |
|  9 | 歪歪       |
| 10 | 丫丫       |
| 11 | 丁丁       |
| 12 | 星星       |
| 13 | 格格       |
| 14 | 张野       |
| 15 | 程咬金     |
| 16 | 程咬银     |
| 17 | 程咬铜     |
| 18 | 程咬铁     |

#2 多条件查询
mysql> select emp_name from employee where post='teacher' and salary>10000;
+----------+
| emp_name |
+----------+
| alex         |
| jinxin     |
+----------+

#3.关键字BETWEEN AND
 SELECT name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

 SELECT name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;

#注意''是空字符串,不是null
 SELECT name,post_comment FROM employee WHERE post_comment='';
 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
mysql>  SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
+------------+---------+
| name       | salary  |
+------------+---------+
| yuanhao    | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
2 rows in set (0.00 sec)

mysql>  SELECT name,salary FROM employee  WHERE salary IN (3000,3500,4000,9000) ;
+------------+---------+
| name       | salary  |
+------------+---------+
| yuanhao    | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
mysql>  SELECT name,salary FROM employee  WHERE salary NOT IN (3000,3500,4000,9000) ;
+-----------+------------+
| name      | salary     |
+-----------+------------+
| egon      |    7300.33 |
| alex      | 1000000.31 |
| wupeiqi   |    8300.00 |
| liwenzhou |    2100.00 |
| jinxin    |   30000.00 |
| xiaomage  |   10000.00 |
| 歪歪      |    3000.13 |
| 丫丫      |    2000.35 |
| 丁丁      |    1000.37 |
| 星星      |    3000.29 |
| 格格      |    4000.33 |
| 张野      |   10000.13 |
| 程咬金    |   20000.00 |
| 程咬银    |   19000.00 |
| 程咬铜    |   18000.00 |
| 程咬铁    |   17000.00 |
+-----------+------------+
16 rows in set (0.00 sec)

#6:关键字LIKE模糊查询
通配符’%’
mysql> SELECT * FROM employee WHERE name LIKE 'jin%';
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
2 rows in set (0.00 sec)


通配符'_'

mysql> SELECT  age FROM employee WHERE name LIKE 'ale_';
+-----+
| age |
+-----+
|  78 |
+-----+
1 row in set (0.00 sec)

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

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

where约束

 

(2)group by 分组查询

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#1、首先明确一点:分组发生在where之后,即分组是基于where之后得到的记录而进行的

#2、分组指的是:将所有记录按照某个相同字段进行归类,比如针对员工信息表的职位分组,或者按照性别进行分组等

#3、为何要分组呢?
    取每个部门的最高工资
    取每个部门的员工数
    取男人数和女人数

小窍门:‘每’这个字后面的字段,就是我们分组的依据

#4、大前提:
    可以按照任意字段分组,但是分组完毕后,比如group by post,只能查看post字段,如果想查看组内信息,需要借助于聚合函数

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当执行以下sql语句的时候,是以post字段查询了组中的第一条数据,没有任何意义,因为我们现在想查出当前组的多条记录。

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mysql> select * from employee group by post;
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name   | sex    | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 14 | 张野   | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 |
|  9 | 歪歪   | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
|  2 | alex   | male   |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  1 | egon   | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
4 rows in set (0.00 sec)

#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果,默认都是组内的第一条记录,但其实这是没有意义的
如果想分组,则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY
mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

#查看MySQL 5.7默认的sql_mode如下:
mysql> select @@global.sql_mode;
+--------------------+
| @@global.sql_mode  |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
1 row in set (0.00 sec)

mysql> exit;#设置成功后,一定要退出,然后重新登录方可生效
Bye

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继续验证通过group by分组之后,只能查看当前字段,如果想查看组内信息,需要借助于聚合函数

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mysql> select * from emp group by post;# 报错
ERROR 1054 (42S22): Unknown column 'post' in 'group statement'



mysql>  select post from employee group by post;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| operation                               |
| sale                                    |
| teacher                                 |
| 老男孩驻沙河办事处外交大使              |
+-----------------------------------------+
4 rows in set (0.00 sec)

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(3)聚合函数

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max()求最大值
min()求最小值
avg()求平均值
sum() 求和
count() 求总个数

#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
# 每个部门有多少个员工
select post,count(id) from employee group by post;
# 每个部门的最高薪水
select post,max(salary) from employee group by post;
# 每个部门的最低薪水
select post,min(salary) from employee group by post;
# 每个部门的平均薪水
select post,avg(salary) from employee group by post;
# 每个部门的所有薪水
select post,sum(age) from employee group by post;

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凤凰彩票平台手机版 31凤凰彩票平台手机版 32

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

小练习

 

 

(4)HAVING过滤

  

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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

凤凰彩票平台手机版 34

 

验证:

凤凰彩票平台手机版 35凤凰彩票平台手机版 36

验证:
mysql> select * from employee where salary>1000000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | alex | male |  78 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
1 row in set (0.00 sec)

mysql> select * from employee having salary>1000000;
ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause

# 必须使用group by才能使用group_concat()函数,将所有的name值连接
mysql> select post,group_concat(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 'post' in 'field list'

View Code

 

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
2. 查询各岗位平均薪资大于10000的岗位名、平均工资
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

 

小练习答案:

凤凰彩票平台手机版 37凤凰彩票平台手机版 38

# 题1:
mysql> select post,group_concat(name),count(id) from employee group by post;
+-----------------------------------------+-----------------------------------------------------------+-----------+
| post                                    | group_concat(name)                                        | count(id) |
+-----------------------------------------+-----------------------------------------------------------+-----------+
| operation                               | 程咬铁,程咬铜,程咬银,程咬金,张野                          |         5 |
| sale                                    | 格格,星星,丁丁,丫丫,歪歪                                  |         5 |
| teacher                                 | xiaomage,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |         7 |
| 老男孩驻沙河办事处外交大使              | egon                                                      |         1 |
+-----------------------------------------+-----------------------------------------------------------+-----------+
4 rows in set (0.00 sec)

mysql> select post,group_concat(name),count(id) from employee group by post having count(id)<2;
+-----------------------------------------+--------------------+-----------+
| post                                    | group_concat(name) | count(id) |
+-----------------------------------------+--------------------+-----------+
| 老男孩驻沙河办事处外交大使              | egon               |         1 |
+-----------------------------------------+--------------------+-----------+
1 row in set (0.00 sec)


#题2:
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+
2 rows in set (0.00 sec)

#题3:
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;
+-----------+--------------+
| post      | avg(salary)  |
+-----------+--------------+
| operation | 16800.026000 |
+-----------+--------------+
1 row in set (0.00 sec)

View Code

 

(5)order by 查询排序

凤凰彩票平台手机版 39

按单列排序
    SELECT * FROM employee ORDER BY age;
    SELECT * FROM employee ORDER BY age ASC;
    SELECT * FROM employee ORDER BY age DESC;
按多列排序:先按照age升序排序,如果年纪相同,则按照id降序
    SELECT * from employee
        ORDER BY age ASC,
        id DESC;

凤凰彩票平台手机版 40

 

凤凰彩票平台手机版 41凤凰彩票平台手机版 42

验证多列排序:
SELECT * from employee ORDER BY age ASC,id DESC;
mysql> SELECT * from employee ORDER BY age ASC,id DESC;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name       | sex    | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 18 | 程咬铁     | female |  18 | 2014-05-12 | operation                               | NULL         |   17000.00 |    403 |         3 |
| 17 | 程咬铜     | male   |  18 | 2015-04-11 | operation                               | NULL         |   18000.00 |    403 |         3 |
| 16 | 程咬银     | female |  18 | 2013-03-11 | operation                               | NULL         |   19000.00 |    403 |         3 |
| 15 | 程咬金     | male   |  18 | 1997-03-12 | operation                               | NULL         |   20000.00 |    403 |         3 |
| 12 | 星星       | female |  18 | 2016-05-13 | sale                                    | NULL         |    3000.29 |    402 |         2 |
| 11 | 丁丁       | female |  18 | 2011-03-12 | sale                                    | NULL         |    1000.37 |    402 |         2 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher                                 | NULL         |   30000.00 |    401 |         1 |
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher                                 | NULL         |    9000.00 |    401 |         1 |
|  1 | egon       | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
| 14 | 张野       | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 |
| 13 | 格格       | female |  28 | 2017-01-27 | sale                                    | NULL         |    4000.33 |    402 |         2 |
|  5 | liwenzhou  | male   |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale                                    | NULL         |    2000.35 |    402 |         2 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
|  8 | xiaomage   | male   |  48 | 2010-11-11 | teacher                                 | NULL         |   10000.00 |    401 |         1 |
|  4 | yuanhao    | male   |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  2 | alex       | male   |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  3 | wupeiqi    | male   |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
18 rows in set (0.01 sec)

mysql>

验证多列排序

 

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 

 

凤凰彩票平台手机版 43凤凰彩票平台手机版 44

# 题目1
select * from employee ORDER BY age asc,hire_date desc;

# 题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+
2 rows in set (0.00 sec)

# 题目3
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| teacher   | 151842.901429 |
| operation |  16800.026000 |
+-----------+---------------+
2 rows in set (0.00 sec)

mysql>

凤凰彩票平台手机版,小练习答案

 

(5)limit  限制查询的记录数:

凤凰彩票平台手机版 45

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

凤凰彩票平台手机版 46

 

小练习:
分页显示,每页5条

凤凰彩票平台手机版 47凤凰彩票平台手机版 48

# 第1页数据
  mysql> select * from  employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name      | sex  | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
|  1 | egon      | male |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
|  2 | alex      | male |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  3 | wupeiqi   | male |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
|  4 | yuanhao   | male |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  5 | liwenzhou | male |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
5 rows in set (0.00 sec)
# 第2页数据
mysql> select * from  employee limit 5,5;
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
|  8 | xiaomage   | male   |  48 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale    | NULL         |  2000.35 |    402 |         2 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec)
# 第3页数据
mysql> select * from  employee limit 10,5;
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| id | name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
| 11 | 丁丁      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |
| 12 | 星星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |
| 13 | 格格      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |
| 14 | 张野      | male   |  28 | 2016-03-11 | operation | NULL         | 10000.13 |    403 |         3 |
| 15 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |
+----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
5 rows in set (0.00 sec)

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编辑:MySQL数据库 本文来源:单表查询

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