mysql学习第7篇:数据库之单表查询

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先创建表

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#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\',\'teacher\',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)
;
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1 1.注意:                                                                                        
2     select * from t1 where 条件 group by 分组字段                                                  
3     1.分组只能查询分组字段,要想查看其余的利用聚合函数                                                               
4     2.聚合函数的分类:count,min,max,avg,group_concat,sum等。                                           
5     3.模糊匹配:用like关键字。                                                                         
6       select * from t1 where name like \'%eg%\'; #%表示任意字符                                      
7       select * from t1 where name like \'d__l\'; #一个下划线表示一个字符,两个下划线就表示两个字符                     
8     4.拷贝表 :create table t2 select * from t1;                                                 
9                create table t2 select * from t1 where 1=2 ;                                  
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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、简单查询

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#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;
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小练习:

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1 查出所有员工的名字,薪资,格式为
    <名字:egon>    <薪资:3000>
select concat(\'<名字:\',name,\'> \'  ,\'<薪资:\',salary,\'>\' ) from employee;
2 查出所有的岗位(去掉重复)
select distinct depart_id from employee;
3 查出所有员工名字,以及他们的年薪,年薪的字段名为年薪
select name,salary*12 年薪 from employee;
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三、where约束

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80或90或100
4. like \'eg%\'
    可以是%或_,
    %表示任意多字符
    _表示一个字符

 like \'e__n\' :
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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#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;
    
#4:关键字IS NULL(判断某个字段是否为NULL不能用等号,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,不是null
    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
    SELECT name,salary FROM employee 
        WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
    SELECT name,salary FROM employee 
        WHERE salary IN (3000,3500,4000,9000) ;

    SELECT name,salary FROM employee 
        WHERE salary NOT IN (3000,3500,4000,9000) ;

#6:关键字LIKE模糊查询
    通配符’%’
    SELECT * FROM employee 
            WHERE name LIKE \'eg%\';

    通配符’_’
    SELECT * FROM employee 
            WHERE name LIKE \'al__\';
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四、having过滤

having和where语法上是一样的。

select * from employee where id>15;    
select * from employee having id>15;   

 

但是having和where不一样的地方在于以下几点!!!

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#!!!执行优先级从高到低:where > group by > 聚合函数 > having >order by
1.where和having的区别                                                                                
     1. Where 是一个约束声明,使用Where约束来自数据库的数据,Where是在结果返回之前起作用的                                        
     (先找到表,按照where的约束条件,从表(文件)中取出数据),Where中不能使用聚合函数                                              
     2.Having是一个过滤声明,是在查询返回结果集以后对查询结果进行的过滤操作                                                     
     (先找到表,按照where的约束条件,从表(文件)中取出数据,然后group by分组,                                                
      如果没有group by则所有记录整体为一组,然后执行聚合函数,然后使用having对聚合的结果进行过滤),                                     
      在Having中可以使用聚合函数。                                                                          
     3.where的优先级比having的优先级高                                                                     
     4.having可以放到group by之后,而where只能放到group by 之前。                                               
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验证不同之处:

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1.查看员工的id>15的有多少个
select count(id) from employee where id>15;#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 id>15; #报错,分析:先执行聚合count(id),后执行having过滤,
                                            #无法对id进行id>15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id>15的过滤,很明显,根本无法获取到id字段
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1 ------having-----------
2 select depart_id,count(id) from employee group by depart_id;
3 select depart_id,count(id) from employee group by depart_id having depart_id = 3;
4 select depart_id,count(id) from employee group by depart_id having count(id)>7;
5 select max(salary) 最大工资 from employee where id>2 group by depart_id having count(id)>3;
6 select * from employee where id>7; #查看所有id>7的员工信息

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
select post,group_concat(name) 员工姓名,count(id) 个数 from employee group by post having count(id)<2;
2. 查询各岗位平均薪资大于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) between 10000 and 20000;

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

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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,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
      select post,group_concat(name) from  employee group by post;#按照岗位分组,并查看组内成员名
      select  post,group_concat(name) as emp_members FROM employee group by post;

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
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强调:

分组:一般相同的多的话就可以分成一组(一定是有重复的字段)
小练习:
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1. 查询岗位名以及岗位包含的所有员工名字
select post,group_concat(name) from employee group by post;
2. 查询岗位名以及各岗位内包含的员工个数 select post,count(id) from employee group by post;
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;
4. 查询岗位名以及各岗位的平均薪资 select post,max(salary) from employee group by post;
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;
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;
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;
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六、关键字的执行优先级(重点)

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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
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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.如果有聚合函数,则将组进行聚合

5.将4的结果过滤:having

6.查出结果:select

7.去重

8.将6的结果按条件排序:order by

9.将7的结果限制显示条数

七、查询排序order by 

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按单列排序
    SELECT * FROM employee ORDER BY salary;
    SELECT * FROM employee ORDER BY salary ASC;
    SELECT * FROM employee ORDER BY salary DESC;

按多列排序:先按照age排序,如果年纪相同,则按照薪资排序
    SELECT * from employee
        ORDER BY age,
        salary DESC;


===========order by========== 1.select * from employee order by salary;#如果不指定,默认就是升序 2.select * from employee order by salary asc; 3.select * from employee order by salary desc; #先按照年龄升序,当年龄相同的太多,分不清大小时,在按照工资降序 4.select * from employee order by age asc, salary desc;
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小例子:

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 * form 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;
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 desc;

八、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

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示例:
    select count(*) from employee;
    select count(*) from employee where depart_id=1;
    select max(salary) from employee;
    select min(salary) from employee;
    select avg(salary) from employee;
    select sum(salary) from  employee;
    select sum(salary) form employee WHERE depart_id=3;
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九、where的补充(使用正则表达式查询)

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1.select * from employee where name regexp \'^ale\';  #匹配以ale开头的员工信息
2.select * from employee where name regexp \'on$\'; #匹配以on结尾的员工信息
3.select * from employee where name regexp \'n{1,2}\'; #匹配name里面包含1到2个n的员工信息
小结:对字符串匹配的方式
where name = \'egon\';
where name like \'yua%\';
where name regexp \'on$\';
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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.*[ng]$\';

十、限制查询的记录数:LIMIT

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=========limit:限制打印几条=========
1.select * from employee limit 3;#打印前三条
2.像这样表示的:指的是从哪开始,往后取几条 (这样的操作一般用来分页)
select * from employee limit 0,3;
select * from employee limit 3,4;
select * from employee limit 6,3;
select * from employee limit 9,3;
3.select * from employee order by id desc limit 3; #查看后三条
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小练习

1. 分页显示,每页5条
select * from employee limit 0,5;
select * from employee limit 5,5;
select * from employee limit 10,5;

 

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