文章目录
数据库的基础操作
mysql采用的同样是sql语法，mysql数据表的常用操作已经总结在这篇文章
创建新的数据库
创建数据库
create database 数据库名 （字符集）
mysql> create database test1 character set utf8 collate utf8_bin;
查看数据库的字符集
show create database 数据库名
mysql> show create database test1
-> ;
+----------+---------------------------------------------------------------------------------+
| Database | Create Database |
+----------+---------------------------------------------------------------------------------+
| test1 | CREATE DATABASE `test1` /*!40100 DEFAULT CHARACTER SET utf8 COLLATE utf8_bin */ |
+----------+---------------------------------------------------------------------------------+
1 row in set (0.00 sec)
查看所有的数据库
show databases;
删除数据库
drop database 数据库名
使用数据库
use database 数据库名字;
查看数据库里的所有表
show tables;
数据表的基础操作
创建表
mysql> create table classes
-> (
-> id int(11) primary key,
-> name varchar(22),
-> age int(11),
-> salary float
-> );
查看表结构
mysql> desc classes;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(22) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| salary | float | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
表中添加新字段
## alter table 表名 add 列名 类型
mysql> alter table classes add location varchar(50);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc classes;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(22) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| salary | float | YES | | NULL | |
| location | varchar(50) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
字段重命名
## alert table 表名 change 原名 新名 类型及约束
mysql> alter table classes change location LOCATION varchar(50);
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc classes;
+----------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(22) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| salary | float | YES | | NULL | |
| LOCATION | varchar(50) | YES | | NULL | |
+----------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
删除字段
alter table 表名 drop 列名
mysql> alter table test3 drop id;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
删除表
drop table 表名
更新数据
语法：
UPDATE table_name SET column1=value1,column2=value2,...WHERE some_column=some_value;
mysql> update classes set name='yu',salary=7200 where id=5;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from classes;
+----+-------+------+--------+----------+
| id | name | age | salary | LOCATION |
+----+-------+------+--------+----------+
| 1 | zhou | 26 | 12000 | aaa |
| 2 | wu | 22 | 9200 | bbb |
| 3 | zheng | 20 | 8500 | ccc |
| 4 | li | 18 | 13850 | ddd |
| 5 | yu | 20 | 7200 | NULL |
| 6 | cao | NULL | NULL | NULL |
+----+-------+------+--------+----------+
6 rows in set (0.00 sec)
插入数据
全列插入：值的顺序与表结构字段的顺序完全一一对应
insert into 表名 values (...)
mysql> insert into class2 values(001,"zhao",1,488);
部分列插入：值的顺序与给出的列顺序对应
insert into 表名 (列1,列2..) values(值1,值2..)
mysql> insert into class(id,name) values(3,"sun");
全列多行插入
insert into 表名 values(...),(...)...;
mysql> insert into class values(4,"cao",20,"aaa",8500),(5,"wei",18,"bbb",)7000),(6,"yang",21,"ccc",11000);
部分列多行插入
insert into 表名(列1,...) values(值1,...),(值1,...)...;
mysql> insert into class(id,name) values(10,"wang"),(11,"wu"),(12,"liu");
主键约束
主键约束要求主键列的数据唯一，并且不允许为空
语法： 字段名 数据类型 PRIMARY KEY [默认值]
-> id INT(11) PRIMARY KEY,
或
在定义完所有列之后指定主键
-> PRIMARY KEY(id)
多字段联合主键
## PRIMARY KEY [字段1，字段2，....]
mysql> create table test2
-> (
-> name varchar(25),
-> de int(11),
-> salary float,
-> primary key(name,de)
-> );
Query OK, 0 rows affected (0.02 sec)
mysql> desc test2;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| name | varchar(25) | NO | PRI | NULL | |
| de | int(11) | NO | PRI | NULL | |
| salary | float | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
外键约束
外键用来在两个表数据之间建立连接，它可以是一列或者多列
目的是一张表insert数据时要受到另一个表的约束
## [CONSTRAINT<外键名>] FOREIGN KEY [字段名1，字段名2...] REFERENCES<主表名> 主键列1[主键列
2...]
# 创建新表与class做约束
mysql> create table class2
-> (
-> id int primary key,
-> name varchar(22),
-> deptID int,
-> score float,
-> constraint ID foreign key(deptID) references class(id)
-> );
Query OK, 0 rows affected (0.01 sec)
#class表插入数据
mysql> select * from class;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 1 | zhao | NULL | NULL | NULL |
| 2 | qian | NULL | NULL | NULL |
| 3 | sun | NULL | NULL | NULL |
+----+------+------+----------+--------+
3 rows in set (0.00 sec)
# class2 再插入数据，class表id不存在的class2不能再写入数据
mysql> insert into class2 values(001,"zhao",1,488);
Query OK, 1 row affected (0.00 sec)
mysql> insert into class2 values(002,"qian",4,375);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test1`.`class2`, CONSTRAINT `ID` FOREIGN KEY (`deptID`) REFERENCES `class` (`id`))
若表已经创建好，需要设置外键
添加约束
alter table class2 add foreign key ID(deptID) references class(id);
删除外键约束
alter table class2 drop foreign key ID;
非空约束
非空约束指字段的值不能为空
not null 是指该字段不能为空，但是可以重复;
unique 是值该字段不可以重复，但是可以为空;
字段名 数据类型 not null
-> name VARCHAR(25) NOT NULL,
[CONSTRATIN <约束名>] UNIQUE （<字段名>）
-> CONSTRAINT STH UNIQUE(name)
## 后期添加约束
mysql> alter table test3 modify deptID int(25) unique;
其中modify的意思可以理解为重构，你甚至可以用modify来进行对其他约束的创建
例如下面：
alter table test modify deptID char(10) not null;建立非空约束
alter table test modify deptID char(10); 删除非空约束
默认约束
默认约束指定某列的默认值。
字段名 数据类型 DEFAULT 默认值
mysql> create table test3
-> (
-> id int primary key,
-> name varchar(25) not null,
-> deptID int(25) default 0001,
-> salary float
-> );
mysql> desc test3
-> ;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(25) | NO | | NULL | |
| deptID | int(25) | YES | | 1 | |
| salary | float | YES | | NULL | |
+--------+-------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
表的属性值自动增加
## 字段名 数据类型 AUTO_INCREMENT
mysql> create table class2
-> (
-> id INT(11) PRIMARY KEY AUTO_INCREMENT,
-> name VARCHAR(25) NOT NULL,
-> deptId INT(11),
-> salary FLOAT
-> );
Query OK, 0 rows affected (0.00 sec)
#若表已经存在
mysql> alter table class2 modify id int(11) auto_increment;
Query OK, 1 row affected (0.04 sec)
Records: 1 Duplicates: 0 Warnings: 0
在表里面插入数据，不写入 id
mysql> insert into class2 (name,score) values('li',96),('liu',63);
Query OK, 2 rows affected (0.00 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> select * from class2
-> ;
+----+------+--------+-------+
| id | name | deptID | score |
+----+------+--------+-------+
| 1 | zhao | 1 | 488 |
| 2 | li | NULL | 96 |
| 3 | liu | NULL | 63 |
+----+------+--------+-------+
3 rows in set (0.00 sec)
更改表的存储引擎
## ALTER TABLE <表名> ENGINE=<更改后的存储引擎>
mysql> alter table classes engine=MyISAM;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table classes\G;
*************************** 1. row ***************************
Table: classes
Create Table: CREATE TABLE `classes` (
`id` int(11) NOT NULL,
`name` varchar(22) COLLATE utf8_bin DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`salary` float DEFAULT NULL,
`LOCATION` varchar(50) COLLATE utf8_bin DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_bin
1 row in set (0.00 sec)
数据表的查询
基本查询
查询所有列
## select * from 表名
mysql> select * from class;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 1 | zhao | NULL | NULL | NULL |
| 2 | qian | NULL | NULL | NULL |
| 3 | sun | NULL | NULL | NULL |
| 4 | cao | 20 | aaa | 8500 |
| 5 | wei | 18 | bbb | 7000 |
| 6 | yang | 21 | ccc | 11000 |
| 10 | wang | NULL | NULL | NULL |
| 11 | wu | NULL | NULL | NULL |
| 12 | liu | NULL | NULL | NULL |
+----+------+------+----------+--------+
9 rows in set (0.01 sec)
查询指定列
## select 列1，列2，...from 表名；
mysql> select id,name from class;
+----+------+
| id | name |
+----+------+
| 1 | zhao |
| 2 | qian |
| 3 | sun |
| 4 | cao |
| 5 | wei |
| 6 | yang |
| 10 | wang |
| 11 | wu |
| 12 | liu |
+----+------+
9 rows in set (0.00 sec)
条件语句查询
where后面支持多种运算符，进行条件的处理 ；
常用的运算符如下：
比较运算符 逻辑运算符 模糊查询 范围查询 空判断
比较运算符如下：
查询class表id大于等于10的人
mysql> select * from class where id >= 10;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 10 | wang | NULL | NULL | NULL |
| 11 | wu | NULL | NULL | NULL |
| 12 | liu | NULL | NULL | NULL |
+----+------+------+----------+--------+
3 rows in set (0.00 sec)
查询class表name是“sun”的人
mysql> select * from class where name="sun";
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 3 | sun | NULL | NULL | NULL |
+----+------+------+----------+--------+
1 row in set (0.00 sec)
逻辑运算符
and 与
or 或
not 否
查询名字是“sun”，并且id小于10
mysql> select * from class where name="sun" and id < 10;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 3 | sun | NULL | NULL | NULL |
+----+------+------+----------+--------+
1 row in set (0.00 sec)
查询名字是“cao”或年龄是18
mysql> select * from class where name="cao" or age=18;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 4 | cao | 20 | aaa | 8500 |
| 5 | wei | 18 | bbb | 7000 |
+----+------+------+----------+--------+
2 rows in set (0.00 sec)
模糊查询
like
%表示任意多个任意字符
_表示一个任意字符
查询名字类似‘ya%’ 或者类似‘li%’
mysql> select * from class where name like 'ya%' or name like 'li%';
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 6 | yang | 21 | ccc | 11000 |
| 12 | liu | NULL | NULL | NULL |
+----+------+------+----------+--------+
2 rows in set (0.00 sec)
范围查询
in表示在一个非连续的范围内
between … and …表示在一个连续的范围内
查询class表id为1 3 5的人
mysql> select * from class where id in(1,3,5);
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 1 | zhao | NULL | NULL | NULL |
| 3 | sun | NULL | NULL | NULL |
| 5 | wei | 18 | bbb | 7000 |
+----+------+------+----------+--------+
3 rows in set (0.00 sec)
查询
mysql> select * from class where id between 3 and 5;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 3 | sun | NULL | NULL | NULL |
| 4 | cao | 20 | aaa | 8500 |
| 5 | wei | 18 | bbb | 7000 |
+----+------+------+----------+--------+
3 rows in set (0.00 sec)
空判断
查询出来条件里的null的项
mysql> select * from class where age is null;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 1 | zhao | NULL | NULL | NULL |
| 2 | qian | NULL | NULL | NULL |
| 3 | sun | NULL | NULL | NULL |
| 10 | wang | NULL | NULL | NULL |
| 11 | wu | NULL | NULL | NULL |
| 12 | liu | NULL | NULL | NULL |
+----+------+------+----------+--------+
6 rows in set (0.00 sec)
也可以反过来查询呐
mysql> select * from class where age is not null;
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 4 | cao | 20 | aaa | 8500 |
| 5 | wei | 18 | bbb | 7000 |
| 6 | yang | 21 | ccc | 11000 |
+----+------+------+----------+--------+
3 rows in set (0.00 sec)
查询排序
将行数据按照列1进行排序，如果某些行 列1 的值相同时，则按照 列2 排序，以此类推
asc从小到大排列，即升序
desc从大到小排序，即降序
默认按照列值从小到大排列（即as升序）
语法使用
select * from 表名 order by 列1 asc|desc [,列2 asc|desc,...]
升序
mysql> select * from classes order by age asc;
+----+-------+------+--------+----------+
| id | name | age | salary | LOCATION |
+----+-------+------+--------+----------+
| 6 | cao | NULL | NULL | NULL |
| 4 | li | 18 | 13850 | ddd |
| 3 | zheng | 20 | 8500 | ccc |
| 5 | yu | 20 | 7200 | NULL |
| 2 | wu | 22 | 9200 | bbb |
| 1 | zhou | 26 | 12000 | aaa |
+----+-------+------+--------+----------+
6 rows in set (0.00 sec)
降序
mysql> select * from classes order by salary desc;
+----+-------+------+--------+----------+
| id | name | age | salary | LOCATION |
+----+-------+------+--------+----------+
| 4 | li | 18 | 13850 | ddd |
| 1 | zhou | 26 | 12000 | aaa |
| 2 | wu | 22 | 9200 | bbb |
| 3 | zheng | 20 | 8500 | ccc |
| 5 | yu | 20 | 7200 | NULL |
| 6 | cao | NULL | NULL | NULL |
+----+-------+------+--------+----------+
6 rows in set (0.00 sec)
limit限制查询
使用limit限制查询结果的数量 select返回所有匹配的行，有可能是表中所有的行，如仅仅需要返回第一行或者前几行，使用limit关键字，语法格式如下：
limit [位置偏移量] 行数
从第5行开始，并且只查询1条记录
mysql> select * from classes limit 5,1;
+----+------+------+--------+----------+
| id | name | age | salary | LOCATION |
+----+------+------+--------+----------+
| 6 | cao | NULL | NULL | NULL |
+----+------+------+--------+----------+
1 row in set (0.00 sec)
聚合函数
总数
count(*) 表示计算总行数，括号中写星与列名，结果是相同的
mysql> select count(*) from classes;
+----------+
| count(*) |
+----------+
| 6 |
+----------+
最大值
max(列) 表示求此列的最大值
mysql> select max(salary) from classes;
+-------------+
| max(salary) |
+-------------+
| 13850 |
+-------------+
1 row in set (0.00 sec)
最小值
mysql> select min(salary) from classes;
+-------------+
| min(salary) |
+-------------+
| 7200 |
+-------------+
1 row in set (0.00 sec)
求和
sum(列) 表示求此列的和
mysql> select sum(salary) from classes;
+-------------+
| sum(salary) |
+-------------+
| 50750 |
+-------------+
1 row in set (0.00 sec)
平均工资
mysql> select sum(salary)/count(*) from classes;
+----------------------+
| sum(salary)/count(*) |
+----------------------+
| 8458.333333333334 |
+----------------------+
1 row in set (0.00 sec)
平均值
mysql> select avg(age) from classes;
+----------+
| avg(age) |
+----------+
| 21.2000 |
+----------+
1 row in set (0.01 sec)
分组查询(group by)
分组查询是对数据按照某个或多个字段进行分组，MySQL中使用group by关键字对数据进行分组，基本语法形式为：
group by 字段 创建分组
Group by 关键字通常和集合函数或group_concat一起使用，例如：
MAX()、MIN()、COUNT()、SUM()、AVG()
按年龄age分组
mysql> select age from classes group by age;
+------+
| age |
+------+
| NULL |
| 18 |
| 20 |
| 22 |
| 26 |
+------+
5 rows in set (0.00 sec)
group by + group_concat()
group by + group_concat(字段名)结合一起使用，
用来查询在该分组下指定字段的集合；
例如：
mysql> select age,group_concat(name) from classes group by age;
+------+--------------------+
| age | group_concat(name) |
+------+--------------------+
| NULL | cao |
| 18 | li |
| 20 | zheng,yu |
| 22 | wu |
| 26 | zhou |
+------+--------------------+
5 rows in set (0.00 sec)
group by + 聚合函数
和group concat()类似，结合函数查询该分组集合的情况
mysql> select age,avg(salary) from classes group by age;
+------+-------------+
| age | avg(salary) |
+------+-------------+
| NULL | NULL |
| 18 | 13850 |
| 20 | 7850 |
| 22 | 9200 |
| 26 | 12000 |
+------+-------------+
5 rows in set (0.00 sec)
mysql>
mysql> select age,count(*) from classes group by age;
+------+----------+
| age | count(*) |
+------+----------+
| NULL | 1 |
| 18 | 1 |
| 20 | 2 |
| 22 | 1 |
| 26 | 1 |
+------+----------+
5 rows in set (0.00 sec)
group by + having
having 条件表达式：用来过滤分组结果
having作用和where类似，但having只能用于group by 而where是用来过滤表数据
如下，查询年龄分组的平均工资且平均超过10000的项
mysql> select age,avg(salary) from classes group by age having avg(salary) > 10000;
+------+-------------+
| age | avg(salary) |
+------+-------------+
| 18 | 13850 |
| 26 | 12000 |
+------+-------------+
2 rows in set (0.00 sec)
mysql>
group by + with rollup
with rollup的作用是：在最后新增一行，来记录当前表中该字段对应的操作结果，即统计记录数量，一般是汇总结果。
mysql> select salary,count(*) from classes group by salary with rollup;
+--------+----------+
| salary | count(*) |
+--------+----------+
| NULL | 1 |
| 7200 | 1 |
| 8500 | 1 |
| 9200 | 1 |
| 12000 | 1 |
| 13850 | 1 |
| NULL | 6 |
+--------+----------+
7 rows in set (0.01 sec)
连接查询
连接是关系数据库模型的主要特点。连接查询是关系数据库中最主要的查询，主要包括内连接、外连接。
内连接
内连接（inner join）使用比较运算符进行表间某些列数据的比较操作，并列出这些表中与连接条件相匹配的数据行，组合成新纪录，也就是说，在内连接查询中，只有满足条件的记录才能出现在结果关系中。
语法：
select * from 表1 inner join 表2 on 表1.列 运算符 表2.列
mysql> select * from class inner join classes on class.name = classes.name;
+----+------+------+----------+--------+----+------+------+--------+----------+
| id | name | age | location | salary | id | name | age | salary | LOCATION |
+----+------+------+----------+--------+----+------+------+--------+----------+
| 4 | cao | 20 | aaa | 8500 | 6 | cao | NULL | NULL | NULL |
| 11 | wu | NULL | NULL | NULL | 2 | wu | 22 | 9200 | bbb |
+----+------+------+----------+--------+----+------+------+--------+----------+
2 rows in set (0.00 sec)
外连接
外连接查询将将查询多个表中相关联的行，返回结果中不仅包含符合连接条件的行，而且还包含左表（左外连接或左连接）、右表（右外连接或右连接），所以说，左连接和右连接都属于外连接，相比内连接时，返回查询结果集合中的仅是符合查询条件和连接条件的行。
语法
select * from 表1 outer join 表2 on 表1.列 运算符 表2.列
左连接
Left join（左连接）：返回包括左表中的所有记录和右表中连接字段相等的记录
mysql> select * from class left (outer) join classes on class.name = classes.name
e;
+----+------+------+----------+--------+------+------+------+--------+----------+
| id | name | age | location | salary | id | name | age | salary | LOCATION |
+----+------+------+----------+--------+------+------+------+--------+----------+
| 11 | wu | NULL | NULL | NULL | 2 | wu | 22 | 9200 | bbb |
| 4 | cao | 20 | aaa | 8500 | 6 | cao | NULL | NULL | NULL |
| 1 | zhao | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
| 2 | qian | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
| 3 | sun | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
| 5 | wei | 18 | bbb | 7000 | NULL | NULL | NULL | NULL | NULL |
| 6 | yang | 21 | ccc | 11000 | NULL | NULL | NULL | NULL | NULL |
| 10 | wang | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
| 12 | liu | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL |
+----+------+------+----------+--------+------+------+------+--------+----------+
9 rows in set (0.00 sec)
右连接
Right join（右连接）：返回包括右表中的所有记录和左表中连接字段相等的记录
mysql> mysql> select class right (outer) join classes on class.name = classes.name;
+------+------+------+----------+--------+----+-------+------+--------+----------+
| id | name | age | location | salary | id | name | age | salary | LOCATION |
+------+------+------+----------+--------+----+-------+------+--------+----------+
| 4 | cao | 20 | aaa | 8500 | 6 | cao | NULL | NULL | NULL |
| 11 | wu | NULL | NULL | NULL | 2 | wu | 22 | 9200 | bbb |
| NULL | NULL | NULL | NULL | NULL | 1 | zhou | 26 | 12000 | aaa |
| NULL | NULL | NULL | NULL | NULL | 3 | zheng | 20 | 8500 | ccc |
| NULL | NULL | NULL | NULL | NULL | 4 | li | 18 | 13850 | ddd |
| NULL | NULL | NULL | NULL | NULL | 5 | yu | 20 | 7200 | NULL |
+------+------+------+----------+--------+----+-------+------+--------+----------+
6 rows in set (0.00 sec)
复合条件连接查询
复合条件连接查询是在连接查询的过程中，通过添加过滤条件，限制查询的结果，使查询的结果更加准确
例如：
mysql> select * from class inner join classes on class.id = classes.id order by class.id desc;
+----+------+------+----------+--------+----+-------+------+--------+----------+
| id | name | age | location | salary | id | name | age | salary | LOCATION |
+----+------+------+----------+--------+----+-------+------+--------+----------+
| 6 | yang | 21 | ccc | 11000 | 6 | cao | NULL | NULL | NULL |
| 5 | wei | 18 | bbb | 7000 | 5 | yu | 20 | 7200 | NULL |
| 4 | cao | 20 | aaa | 8500 | 4 | li | 18 | 13850 | ddd |
| 3 | sun | NULL | NULL | NULL | 3 | zheng | 20 | 8500 | ccc |
| 2 | qian | NULL | NULL | NULL | 2 | wu | 22 | 9200 | bbb |
| 1 | zhao | NULL | NULL | NULL | 1 | zhou | 26 | 12000 | aaa |
+----+------+------+----------+--------+----+-------+------+--------+----------+
6 rows in set (0.00 sec)
mysql>
mysql> select * from class inner join classes on class.id = classes.id and class.id=2;
+----+------+------+----------+--------+----+------+------+--------+----------+
| id | name | age | location | salary | id | name | age | salary | LOCATION |
+----+------+------+----------+--------+----+------+------+--------+----------+
| 2 | qian | NULL | NULL | NULL | 2 | wu | 22 | 9200 | bbb |
+----+------+------+----------+--------+----+------+------+--------+----------+
1 row in set (0.00 sec)
子查询
子查询指一个查询语句嵌套在另一个查询语句内部的查询，这个特性从mysql4.1开始引入。在select子句中先计算子查询，子查询结果作为外层另一个查询的过滤条件，查询可以基于一个表或者多个表。子查询中常用的操
作符有any（some）、all、in、exists。子查询可以添加到select、update和delete语句中，而且可以进行多层嵌套。子查询中也可以使用比较运算符，如“<”,“<=”,“>”,“>=”和“!=”等。
带any、some关键字的子查询
any和some关键字是同义词，表示满足其中任一条件，它们允许创建一个表达式对子查询的返回值列进行比较，只要满足内层子查询中的任何一个比较条件，就返回一个结果作为外层查询的条件。
例：
先创建2个新表再插入数据
mysql> create table tb1(num1 int not null);
Query OK, 0 rows affected (0.03 sec)
mysql> create table tb2(num2 int not null);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into tb1 values(1),(5),(13),(27);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> insert into tb2 values(6),(14),(11),(20);
Query OK, 4 rows affected (0.01 sec)
Records: 4 Duplicates: 0 Warnings: 0
查询tb1表里面有大于tb2的值
mysql> select num1 from tb1 where num1 > any (select num2 from tb2);
+------+
| num1 |
+------+
| 13 |
| 27 |
+------+
2 rows in set (0.01 sec)
带all关键字的子查询
all关键字与any和some不同，使用all时需要同时满足所有内层查询的条件
查询tb1里面的值都大于tb2的值
mysql> select num1 from tb1 where num1 > all (select num2 from tb2);
+------+
| num1 |
+------+
| 27 |
+------+
1 row in set (0.00 sec)
正则表达式查询
正则表达式通常被用来检索或替换那些符合某个模式的文本内容，根据指定的匹配模式匹配文本中符合要求的特殊字符串；
mysql中使用regexp关键字指定正则表达式的字符匹配模式.
正则选项参考如下表：
例子：
mysql> select * from classes where name regexp '^l';
+----+------+------+--------+----------+
| id | name | age | salary | LOCATION |
+----+------+------+--------+----------+
| 4 | li | 18 | 13850 | ddd |
+----+------+------+--------+----------+
1 row in set (0.00 sec)
mysql> select * from class where name regexp 'ya.*|wa.*';
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 6 | yang | 21 | ccc | 11000 |
| 10 | wang | NULL | NULL | NULL |
+----+------+------+----------+--------+
2 rows in set (0.00 sec)
mysql> select * from class where location regexp 'b{2,}';
+----+------+------+----------+--------+
| id | name | age | location | salary |
+----+------+------+----------+--------+
| 5 | wei | 18 | bbb | 7000 |
+----+------+------+----------+--------+
1 row in set (0.00 sec)