MySQL数据库----单表查询

先创建表

#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)
;

1.注意：
    select * from t1 where 条件 group by 分组字段
    1.分组只能查询分组字段，要想查看其余的利用聚合函数
    2.聚合函数的分类：count,min,max,avg,group_concat,sum等。
    3.模糊匹配：用like关键字。
      select * from t1 where name like '%eg%'; #%表示任意字符
      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
    4.拷贝表 ：create table t2 select * from t1;
               create table t2 select * from t1 where 1=2 ;                                  

一点小知识复习

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

#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>
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;

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

#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__';

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！！！

#！！！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。                                               

验证不同之处：

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

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

having 举例

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

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

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：

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;

六、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

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

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;

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

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;

九、where的补充（使用正则表达式查询）

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

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

=========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

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;