数据操作

一介绍

MySQL数据操作： DML

========================================================

在MySQL管理软件中，可以通过SQL语句中的DML语言来实现数据的操作，包括：

使用INSERT实现数据的插入
UPDATE实现数据的更新
使用DELETE实现数据的删除
使用SELECT查询数据。

========================================================

本节内容包括：

插入数据
更新数据
删除数据
查询数据

二插入数据INSERT

1. 插入完整数据（顺序插入）
    语法一：
    INSERT INTO 表名(字段1,字段2,字段3…字段n) VALUES(值1,值2,值3…值n);

    语法二：
    INSERT INTO 表名 VALUES (值1,值2,值3…值n);

2. 指定字段插入数据
    语法：
    INSERT INTO 表名(字段1,字段2,字段3…) VALUES (值1,值2,值3…);

3. 插入多条记录
    语法：
    INSERT INTO 表名 VALUES
        (值1,值2,值3…值n),
        (值1,值2,值3…值n),
        (值1,值2,值3…值n);
        
4. 插入查询结果
    语法：
    INSERT INTO 表名(字段1,字段2,字段3…字段n) 
                    SELECT (字段1,字段2,字段3…字段n) FROM 表2
                    WHERE …;

示例：

create table user(host char(60),user char(16)) select host,user from mysql.user
desc user;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| host  | char(60) | YES  |     | NULL    |       |
| user  | char(16) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

select * from user;
+-----------+------+
| host      | user |
+-----------+------+
| %         | alex |
| %         | egon |
| 127.0.0.1 | root |
| ::1       | root |
| localhost |      |
| localhost | root |
+-----------+------+

create table user_new1(ip char(60),username char(16)) select host as ip,user as username from mysql.user;
desc user_new1;
+----------+----------+------+-----+---------+-------+
| Field    | Type     | Null | Key | Default | Extra |
+----------+----------+------+-----+---------+-------+
| ip       | char(60) | YES  |     | NULL    |       |
| username | char(16) | YES  |     | NULL    |       |
+----------+----------+------+-----+---------+-------+
select * from user_new1;
+-----------+----------+
| ip        | username |
+-----------+----------+
| %         | alex     |
| %         | egon     |
| 127.0.0.1 | root     |
| ::1       | root     |
| localhost |          |
| localhost | root     |
+-----------+----------+

select host,user from mysql.user;
+-----------+------+
| host      | user |
+-----------+------+
| %         | alex |
| %         | egon |
| 127.0.0.1 | root |
| ::1       | root |
| localhost |      |
| localhost | root |
+-----------+------+
select host as 主机名,user as 用户名 from mysql.user;
+-----------+-----------+
| 主机名    | 用户名    |
+-----------+-----------+
| %         | alex      |
| %         | egon      |
| 127.0.0.1 | root      |
| ::1       | root      |
| localhost |           |
| localhost | root      |
+-----------+-----------+

select host 主机名,user 用户名 from mysql.user;
+-----------+-----------+
| 主机名    | 用户名    |
+-----------+-----------+
| %         | alex      |
| %         | egon      |
| 127.0.0.1 | root      |
| ::1       | root      |
| localhost |           |
| localhost | root      |
+-----------+-----------+

View Code

三更新数据UPDATE

语法：
    UPDATE 表名 SET
        字段1=值1,
        字段2=值2,
        WHERE CONDITION;

示例：
    UPDATE mysql.user SET password=password(‘123’) 
        where user=’root’ and host=’localhost’;

四删除数据DELETE

语法：
    DELETE FROM 表名 
        WHERE CONITION;

示例：
    DELETE FROM mysql.user 
        WHERE password=’’;
    delete from 表 where id >= 20;

清空表
truncate 表;

五查询数据SELECT

　　1.单表查询

　　2.多表查询

单表查询

一单表查询的语法

select 字段1，字段2，字段3。。。。 from 表名
    where 约束条件
    group by 分组的字段
    having 过滤条件
    order by 排序字段
    limit 限制条件;

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

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.将分组的结果进行having过滤

5.执行select

6.去重

7.将结果按条件排序：order by

8.限制结果的显示条数

三简单查询

准备表和记录

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)
;

#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

准备表和记录

View Code

#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;

练习：

select name,salary from employee;
+------------+------------+
| name       | salary     |
+------------+------------+
| egon       |    7300.33 |
| alex       | 1000000.31 |
| wupeiqi    |    8300.00 |
| yuanhao    |    3500.00 |
| liwenzhou  |    2100.00 |
| jingliyang |    9000.00 |
| jinxin     |   30000.00 |
| 成龙       |   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 |
+------------+------------+

select post from employee;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| 老男孩驻沙河办事处外交大使              |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| sale                                    |
| sale                                    |
| sale                                    |
| sale                                    |
| sale                                    |
| operation                               |
| operation                               |
| operation                               |
| operation                               |
| operation                               |
+-----------------------------------------+

select distinct post from employee;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| 老男孩驻沙河办事处外交大使              |
| teacher                                 |
| sale                                    |
| operation                               |
+-----------------------------------------+

select name,salary*12 from employee;
+------------+-------------+
| name       | salary*12   |
+------------+-------------+
| egon       |    87603.96 |
| alex       | 12000003.72 |
| wupeiqi    |    99600.00 |
| yuanhao    |    42000.00 |
| liwenzhou  |    25200.00 |
| jingliyang |   108000.00 |
| jinxin     |   360000.00 |
| 成龙       |   120000.00 |
| 歪歪       |    36001.56 |
| 丫丫       |    24004.20 |
| 丁丁       |    12004.44 |
| 星星       |    36003.48 |
| 格格       |    48003.96 |
| 张野       |   120001.56 |
| 程咬金     |   240000.00 |
| 程咬银     |   228000.00 |
| 程咬铜     |   216000.00 |
| 程咬铁     |   204000.00 |
+------------+-------------+

select name,salary*12 as annual_salary from employee;    #as可以省略
select name,salary*12 annual_salary from employee;
+------------+---------------+
| name       | annual_salary |
+------------+---------------+
| egon       |      87603.96 |
| alex       |   12000003.72 |
| wupeiqi    |      99600.00 |
| yuanhao    |      42000.00 |
| liwenzhou  |      25200.00 |
| jingliyang |     108000.00 |
| jinxin     |     360000.00 |
| 成龙       |     120000.00 |
| 歪歪       |      36001.56 |
| 丫丫       |      24004.20 |
| 丁丁       |      12004.44 |
| 星星       |      36003.48 |
| 格格       |      48003.96 |
| 张野       |     120001.56 |
| 程咬金     |     240000.00 |
| 程咬银     |     228000.00 |
| 程咬铜     |     216000.00 |
| 程咬铁     |     204000.00 |
+------------+---------------+

SELECT CONCAT('姓名: ',name,'  年薪: ', salary*12)  AS Annual_salary FROM employee;
+---------------------------------------+
| Annual_salary                         |
+---------------------------------------+
| 姓名: egon  年薪: 87603.96            |
| 姓名: alex  年薪: 12000003.72         |
| 姓名: wupeiqi  年薪: 99600.00         |
| 姓名: yuanhao  年薪: 42000.00         |
| 姓名: liwenzhou  年薪: 25200.00       |
| 姓名: jingliyang  年薪: 108000.00     |
| 姓名: jinxin  年薪: 360000.00         |
| 姓名: 成龙  年薪: 120000.00           |
| 姓名: 歪歪  年薪: 36001.56            |
| 姓名: 丫丫  年薪: 24004.20            |
| 姓名: 丁丁  年薪: 12004.44            |
| 姓名: 星星  年薪: 36003.48            |
| 姓名: 格格  年薪: 48003.96            |
| 姓名: 张野  年薪: 120001.56           |
| 姓名: 程咬金  年薪: 240000.00         |
| 姓名: 程咬银  年薪: 228000.00         |
| 姓名: 程咬铜  年薪: 216000.00         |
| 姓名: 程咬铁  年薪: 204000.00         |
+---------------------------------------+

SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary FROM employee;
+----------------------+
| Annual_salary        |
+----------------------+
| egon:87603.96        |
| alex:12000003.72     |
| wupeiqi:99600.00     |
| yuanhao:42000.00     |
| liwenzhou:25200.00   |
| jingliyang:108000.00 |
| jinxin:360000.00     |
| 成龙:120000.00       |
| 歪歪:36001.56        |
| 丫丫:24004.20        |
| 丁丁:12004.44        |
| 星星:36003.48        |
| 格格:48003.96        |
| 张野:120001.56       |
| 程咬金:240000.00     |
| 程咬银:228000.00     |
| 程咬铜:216000.00     |
| 程咬铁:204000.00     |
+----------------------+

View Code

四 WHERE约束

where字句中可以使用：

　　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80或90或100
　　4. like 'egon%'
　　pattern可以是%或_，
　　%表示任意多字符
　　_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

#1:单条件查询
select id,name from employee 
where post='teacher';

#2:多条件查询
SELECT name,salary FROM employee WHERE post='teacher' AND salary>10000;
select id,name from employee where id >=3 and id < 5;
select id,name from employee where id <=3 or id >= 5;

#3:关键字BETWEEN AND
select id,name from employee where id between 3 and 5;
#相当于：
select id,name from employee where id >=3 and id <= 5;

#4:关键字IN集合查询
select id,name,age from employee 
where age not in (60,70,80,18,'aaa');# 可以有字符串
#语法上比下面的会简单很多：
select id,name,age from employee 
where age = 60 or age = 70 or age = 80 or age = 18;

#5:关键字LIKE模糊查询
通配符’%’
select id,name,age from employee where name like 'e%';
通配符’_’
select id,name,age from employee where name not like 'ale_';

#6: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 * from employee where not post_comment is null;
SELECT name,post_comment FROM employee WHERE post_comment IS NOT NULL;

View Code

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

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 and 10000;
4.
select * from employee where not post_comment is 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 employee where post = 'teacher' and name like 'jin%';

View Code

五分组查询:GROUP BY

1 什么是分组？为什么要分组？

#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

示例：

#统计：每个部门的员工数
select post,count(id) from employee group by post;

#强调：
#1.分组之后，select只能查看到分组的字段，要想查组内内容
#不能直接查看，需要借助于聚合函数max，min，avg，sum，count
#2.分组的目的是为类以组为单位来处理记录，而不是处理单独的记录
#3.如果我们用unique的字段作为分组的依据，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

#查询总员工数:没有分组，默认整体一组
select count(id) from employee;
#查看每个部门的员工数
select post,count(id) from employee group by post;
#查看teacher部门的员工数
select count(id) from employee where post='teacher';

#聚合函数的使用：
select post,max(salary) from employee group by post;
select post,min(salary) from employee group by post;
select post,avg(salary) from employee group by post;
select post,sum(salary) from employee group by post;
select post,count(id) from employee group by post;
select post,group_concat(name) from employee group by post;
select post,group_concat('薪资',salary) from employee group by post;

View Code

2 ONLY_FULL_GROUP_BY

sql_mode设置

select @@global.sql_mode;
+------------------------+
| @@global.sql_mode      |
+------------------------+
| NO_ENGINE_SUBSTITUTION |
+------------------------+
select post from employee group by post;    #可以执行
select post,name from employee group by post;    #可以执行


set global sql_mode='ONLY_FULL_GROUP_BY';
select @@global.sql_mode;
+--------------------+
| @@global.sql_mode  |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
select post from employee group by post;    #可以执行
select post,name from employee group by post;    #报错
ERROR 1055 (42000): 'day044.employee.name' isn't in GROUP BY

#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：
set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
退出客户端，重新进入。
select @@global.sql_mode;
STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
select post from employee group by post;    #可以执行
select post,name from employee group by post;    #可以执行

View Code

示例：

#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
select post,name from employee group by post;
+-----------------------------------------+--------+
| post                                    | name   |
+-----------------------------------------+--------+
| operation                               | 张野   |
| sale                                    | 歪歪   |
| teacher                                 | alex   |
| 老男孩驻沙河办事处外交大使              | egon   |
+-----------------------------------------+--------+

set global sql_mode='ONLY_FULL_GROUP_BY';
#设置成功后，一定要退出，然后重新登录方可生效
select post,name from employee group by post;
ERROR 1055 (42000): 'day044.employee.name' isn't in GROUP BY

View Code

3 GROUP BY

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：

　　如果我们用unique的字段作为分组的依据，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。

4 聚合函数

#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

示例：
    SELECT COUNT(*) FROM employee;
    相当于：
    SELECT COUNT(id) FROM employee;
    SELECT COUNT(*) FROM employee WHERE depart_id=1;
    相当于：
    SELECT COUNT(id) 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) FROM employee WHERE depart_id=3;

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

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

六 HAVING过滤

　　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

代码示例：

# 一下都是设置了set global sql_mode='ONLY_FULL_GROUP_BY';
select * from employee where salary > 10000;    
#取出员工数大于3的部门
select post from employee group by post having count(id) > 3;
#取出id是大于10的,员工数大于3的部门
select post from employee where id > 10 group by post having count(id) > 3;


#错误示范。错误：having是在分组之后的，意味着，只能用分组的字段或者聚合函数作为过滤条件
select post from employee
    group by post
    having salary > 10000;    #不做第一步设置就可以。
select * from employee
    having salary > 10000;      #不做第一步设置就可以。

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

1.
select post,group_concat(name),count(id) from employee 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

七去重：DISTINCT

select distinct 字段1，字段2，字段3。。。。 from 表名
    where 约束条件
    group by 分组的字段
    having 过滤条件
    order by 排序字段
    limit 限制条件;

示例：

select distinct post from employee;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| 老男孩驻沙河办事处外交大使              |
| teacher                                 |
| sale                                    |
| operation                               |
+-----------------------------------------+

select post from employee;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| 老男孩驻沙河办事处外交大使              |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| teacher                                 |
| sale                                    |
| sale                                    |
| sale                                    |
| sale                                    |
| sale                                    |
| operation                               |
| operation                               |
| operation                               |
| operation                               |
| operation                               |
+-----------------------------------------+

View Code

八查询排序:ORDER BY

按单列排序
    升序：
    select * from employee order by age;
    select * from employee order by age asc;
    降序：
    select * from employee order by age 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;

示例：

select post,max(salary) from employee group by post order by max(salary);
select post,max(salary) as m from employee group by post order by m;
# 按照执行顺序，select在order by之前，也就是说，需要的表的内容已经确定下来，只是利用order by给其排个序。（默认升序）

看下面这个例子：
#取出平均工资>10000的部门以及它的平均工资
select post,avg(salary) from employee group by post having avg(salary) > 10000;
#错误，执行having时，select还没有执行呢
select post,avg(salary) as x from employee group by post having x > 10000;
ERROR 1463 (42000): Non-grouping field 'x' is used in HAVING clause

View Code

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

1.
select * from employee ORDER BY age asc,hire_date desc;
2.
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) asc;
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary);    #默认升序
3.
select post,avg(salary) from employee group by post having avg(salary) > 10000 order by avg(salary) desc;

View Code

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

示例：

#取出工资最高的那个员工信息
select * from employee order by salary desc limit 1;

View Code

练习：

1. 分页显示，每页3条

select * from employee limit 0,3;
select * from employee limit 3,3;
select * from employee limit 6,3;

View Code

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$';

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]$';

View Code

posted @ 2017-10-25 17:55 王庆帅阅读(73) 评论(0) 编辑收藏举报

刷新页面返回顶部

王庆帅

数据操作

数据操作

一 介绍

二 插入数据INSERT

三 更新数据UPDATE

四 删除数据DELETE

五 查询数据SELECT

单表查询

一 单表查询的语法

二 关键字的执行优先级(重点)

三 简单查询

四 WHERE约束

五 分组查询:GROUP BY

六 HAVING过滤

七 去重：DISTINCT

八 查询排序:ORDER BY

九 限制查询的记录数:LIMIT

十 使用正则表达式查询

公告

一介绍

二插入数据INSERT

三更新数据UPDATE

四删除数据DELETE

五查询数据SELECT

一单表查询的语法

二关键字的执行优先级(重点)

三简单查询

五分组查询:GROUP BY

七去重：DISTINCT

八查询排序:ORDER BY

九限制查询的记录数:LIMIT

十使用正则表达式查询