day44 约束条件、表之间是关系，记录的相关操作

tech2022-11-26 91

一、约束条件1.1、介绍1.2、not null与default1.3、unique1.4、主键 primary key1.5、auto_increment1.6、 foreign key 二、表之间的三种关系2.1、如何找出两张表之间的关系2.2、建立表之间的关系2.2.1、多对一2.2.2、多对多2.3.3、一对一三、记录的相关操作3.1、插入数据INSERT3.2、更新数据UPDATE3.3、删除数据DELETE3.4、查询数据SELECT3.4.1、单表查询3.4.2、简单查询

一、约束条件

1.1、介绍

约束条件与数据类型的宽度一样，都是可选参数

作用：用于保证数据的完整性和一致性主要分为：

PRIMARY KEY (PK) 标识该字段为该表的主键，可以唯一的标识记录 FOREIGN KEY (FK) 标识该字段为该表的外键 NOT NULL 标识该字段不能为空 UNIQUE KEY (UK) 标识该字段的值是唯一的 AUTO_INCREMENT 标识该字段的值自动增长（整数类型，而且为主键） DEFAULT 为该字段设置默认值 UNSIGNED 无符号 ZEROFILL 使用0填充

说明：

1. 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值 2. 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值 sex enum('male','female') not null default 'male' age int unsigned NOT NULL default 20 必须为正值（无符号）不允许为空默认是20 3. 是否是key 主键 primary key 外键 foreign key 索引 (index,unique...)

1.2、not null与default

是否可空，null表示空，非字符串 not null - 不可空 null - 可空

默认值，创建列时可以指定默认值，当插入数据时如果未主动设置，则自动添加默认值 create table tb1( nid int not null defalut 2, num int not null )

验证

==================not null==================== mysql> create table t1(x int not null);#设置字段id不为空 Query OK, 0 rows affected (0.05 sec) mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | x | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec) mysql> insert t1 values();#不能插入空 ERROR 1364 (HY000): Field 'x' doesn't have a default value mysql> create table t2(x int);#id字段默认可以插入空 Query OK, 0 rows affected (0.03 sec) mysql> desc t2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | x | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.01 sec) mysql> insert t2 values();#可以插入空 Query OK, 1 row affected (0.01 sec) ==================default==================== #设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值 mysql> create table t3(id int default 1); mysql> desc t3; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | x | int(11) | YES | | 1 | | +-------+---------+------+-----+---------+-------+ 1 row in set (0.00 sec) mysql> insert t3 values(); mysql> select x from t3; +------+ | x | +------+ | 1 | +------+ 1 row in set (0.00 sec) mysql> alter table t3 modify id int not null default 1; mysql> insert t3 values(); #空，插入空默认填入default指定的默认值 Query OK, 1 row affected (0.00 sec) mysql> select x from t3; # 空，插入空默认填入default指定的默认值 +---+ | x | +---+ | 1 | | 1 | +---+

1.3、unique

单列唯一

mysql> create table t3(name varchar(10) unique); Query OK, 0 rows affected (0.05 sec) mysql> insert into t3 values("egon"); Query OK, 1 row affected (0.01 sec) mysql> insert into t3 values("tom"); Query OK, 1 row affected (0.01 sec) mysql> insert into t3 values("egon");#报错 ERROR 1062 (23000): Duplicate entry 'egon' for key 'name'

联合唯一

mysql> create table server(id int,name varchar(10),ip varchar(15),port int,unique(ip,port),unique(name)); Query OK, 0 rows affected (0.04 sec) mysql> insert server values (1,'web1','10.10.0.11',8080); Query OK, 1 row affected (0.01 sec) mysql> insert server values (1,'web2','10.10.0.11',8081); Query OK, 1 row affected (0.00 sec) mysql> insert server values (3,'web3','10.10.0.11',8081);#报错 ERROR 1062 (23000): Duplicate entry '10.10.0.11-8081' for key 'ip' mysql>

not null 和unique的化学反应=>会被识别成表的主键

1.4、主键 primary key

从约束角度看primary key字段的值不为空且唯一，那我们直接使用not null+unique不就可以了吗，要它干什么？

主键primary key是innodb存储引擎组织数据的依据，innodb称之为索引组织表，一张表中必须有且只有一个主键。

一个表中可以：

单列做主键多列做主键（复合主键）

单列主键

多列主键（了解）

create table t7( id int, name varchar(5), primary key(id,name) ); mysql> desc t7; +-------+------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(5) | NO | PRI | NULL | | +-------+------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)

1.5、auto_increment

约束字段为自动增长，被约束的字段必须同时被key约束

#不指定id，则自动增长 mysql> create table student( -> id int primary key auto_increment, -> name varchar(20), -> sex enum('male','female') default 'male' -> ); Query OK, 0 rows affected (0.04 sec) mysql> desc student; +-------+-----------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+-----------------------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | varchar(20) | YES | | NULL | | | sex | enum('male','female') | YES | | male | | +-------+-----------------------+------+-----+---------+----------------+ 3 rows in set (0.00 sec) mysql> insert into student(name) values ('egon'),('alex'); Query OK, 2 rows affected (0.01 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | | 2 | alex | male | +----+------+------+ 2 rows in set (0.00 sec) #也可以指定id mysql> insert into student values(4,'asb','female'); Query OK, 1 row affected (0.00 sec) mysql> insert into student values(7,'wsb','female'); Query OK, 1 row affected (0.00 sec) mysql> select * from student; +----+------+--------+ | id | name | sex | +----+------+--------+ | 1 | egon | male | | 2 | alex | male | | 4 | asb | female | | 7 | wsb | female | +----+------+--------+ #对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长 mysql> delete from student; Query OK, 4 rows affected (0.00 sec) mysql> select * from student; Empty set (0.00 sec) mysql> insert into student(name) values('ysb'); mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 8 | ysb | male | +----+------+------+ #应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它 mysql> truncate student; Query OK, 0 rows affected (0.01 sec) mysql> insert into student(name) values('egon'); Query OK, 1 row affected (0.01 sec) mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | +----+------+------+ 1 row in set (0.00 sec)

了解知识点

#在创建完表后，修改自增字段的起始值 mysql> create table student( -> id int primary key auto_increment, -> name varchar(20), -> sex enum('male','female') default 'male' -> ); mysql> alter table student auto_increment=3; mysql> show create table student; ....... ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8 mysql> insert into student(name) values('egon'); Query OK, 1 row affected (0.01 sec) mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 3 | egon | male | +----+------+------+ row in set (0.00 sec) mysql> show create table student; ....... ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 #也可以创建表时指定auto_increment的初始值，注意初始值的设置为表选项，应该放到括号外 create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' )auto_increment=3; #设置步长 sqlserver：自增步长基于表级别 create table t1（ id int。。。）engine=innodb,auto_increment=2 步长=2 default charset=utf8 mysql自增的步长： show session variables like 'auto_inc%'; #基于会话级别 set session auth_increment_increment=2 #修改会话级别的步长 #基于全局级别的 set global auth_increment_increment=2 #修改全局级别的步长（所有会话都生效） #！！！注意了注意了注意了！！！ If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored. 翻译：如果auto_increment_offset的值大于auto_increment_increment的值，则auto_increment_offset的值会被忽略，这相当于第一步步子就迈大了，扯着了蛋比如：设置auto_increment_offset=3，auto_increment_increment=2 mysql> set global auto_increment_increment=5; Query OK, 0 rows affected (0.00 sec) mysql> set global auto_increment_offset=3; Query OK, 0 rows affected (0.00 sec) mysql> show variables like 'auto_incre%'; #需要退出重新登录 +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | | auto_increment_offset | 1 | +--------------------------+-------+ create table student( id int primary key auto_increment, name varchar(20), sex enum('male','female') default 'male' ); mysql> insert into student(name) values('egon1'),('egon2'),('egon3'); mysql> select * from student; +----+-------+------+ | id | name | sex | +----+-------+------+ | 3 | egon1 | male | | 8 | egon2 | male | | 13 | egon3 | male | +----+-------+------+

1.6、 foreign key

表之间在逻辑上有关联，但在物理层面上没有硬性关联

fk 建立表之间在物理层面上的关联

例：

员工信息表有三个字段：工号姓名部门

公司有3个部门，但是有1个亿的员工，那意味着部门这个字段需要重复存储，部门名字越长，越浪费

解决方法：

我们完全可以定义一个部门表

然后让员工信息表关联该表，如何关联，即foreign key

#表类型必须是innodb存储引擎，且被关联的字段，即references指定的另外一个表的字段，必须保证唯一 # 先创建被关联表 create table department( id int primary key, name varchar(20) not null )engine=innodb; #dpt_id外键，关联父表（department主键id），同步更新，同步删除 # 再创建关联表 create table employee( id int primary key, name varchar(20) not null, dpt_id int, constraint fk_name foreign key(dpt_id) references department(id) on delete cascade on update cascade )engine=innodb; #先往父表department中插入记录 insert into department values (1,'欧德博爱技术有限事业部'), (2,'艾利克斯人力资源部'), (3,'销售部'); #再往子表employee中插入记录 insert into employee values (1,'egon',1), (2,'alex1',2), (3,'alex2',2), (4,'alex3',2), (5,'李坦克',3), (6,'刘飞机',3), (7,'张火箭',3), (8,'林子弹',3), (9,'加特林',3) ; #删父表department，子表employee中对应的记录跟着删 mysql> delete from department where id=3; mysql> select * from employee; +----+-------+--------+ | id | name | dpt_id | +----+-------+--------+ | 1 | egon | 1 | | 2 | alex1 | 2 | | 3 | alex2 | 2 | | 4 | alex3 | 2 | +----+-------+--------+ #更新父表department，子表employee中对应的记录跟着改 mysql> update department set id=22222 where id=2; mysql> select * from employee; +----+-------+--------+ | id | name | dpt_id | +----+-------+--------+ | 1 | egon | 1 | | 3 | alex2 | 22222 | | 4 | alex3 | 22222 | | 5 | alex1 | 22222 | +----+-------+--------+

二、表之间的三种关系

2.1、如何找出两张表之间的关系

分析步骤： #1、先站在左表的角度去找是否左表的多条记录可以对应右表的一条记录，如果是，则证明左表的一个字段foreign key 右表一个字段（通常是id）

#2、再站在右表的角度去找是否右表的多条记录可以对应左表的一条记录，如果是，则证明右表的一个字段foreign key 左表一个字段（通常是id）

#3、总结： #多对一：如果只有步骤1成立，则是左表多对一右表如果只有步骤2成立，则是右表多对一左表

#多对多如果步骤1和2同时成立，则证明这两张表时一个双向的多对一，即多对多,需要定义一个这两张表的关系表来专门存放二者的关系

#一对一: 如果1和2都不成立，而是左表的一条记录唯一对应右表的一条记录，反之亦然。这种情况很简单，就是在左表foreign key右表的基础上，将左表的外键字段设置成unique即可

2.2、建立表之间的关系

2.2.1、多对一

’‘’ 一对多或称为多对一三张表：出版社，作者信息，书一对多（或多对一）：一个出版社可以出版多本书关联方式：foreign key ’‘’ create table press( id int primary key auto_increment, name varchar(20) ); create table book( id int primary key auto_increment, name varchar(20), press_id int not null, foreign key(press_id) references press(id) on delete cascade on update cascade ); insert into press(name) values ('北京工业地雷出版社'), ('人民音乐不好听出版社'), ('知识产权没有用出版社') ; mysql> select * from press; +----+--------------------------------+ | id | name | +----+--------------------------------+ | 1 | 北京工业地雷出版社 | | 2 | 人民音乐不好听出版社 | | 3 | 知识产权没有用出版社 | +----+--------------------------------+ 3 rows in set (0.00 sec) insert into book(name,press_id) values ('九阳神功',1), ('九阴真经',2), ('九阴白骨爪',2), ('独孤九剑',3), ('降龙十巴掌',2), ('葵花宝典',3) ; mysql> select * from book; +----+-----------------+----------+ | id | name | press_id | +----+-----------------+----------+ | 1 | 九阳神功 | 1 | | 2 | 九阴真经 | 2 | | 3 | 九阴白骨爪 | 2 | | 4 | 独孤九剑 | 3 | | 5 | 降龙十巴掌 | 2 | | 6 | 葵花宝典 | 3 | +----+-----------------+----------+ 6 rows in set (0.00 sec)

2.2.2、多对多

’‘’ 三张表：出版社，作者信息，书多对多：一个作者可以写多本书，一本书也可以有多个作者，双向的一对多，即多对多　　关联方式：foreign key+一张新的表 ‘’‘ create table author( id int primary key auto_increment, name varchar(10) ); create table books( id int primary key auto_increment, name varchar(16) ); create table author2book( id int primary key auto_increment, author_id int, book_id int, foreign key(author_id) references author(id) on delete cascade on update cascade, foreign key(book_id) references book(id) on delete cascade on update cascade );

2.3.3、一对一

’‘’ 两张表：学生表和客户表一对一：一个学生是一个客户，一个客户有可能变成一个学校，即一对一的关系关联方式：foreign key+unique ‘’‘ create table customer( id int primary key auto_increment, name varchar(16), phone char(11) ); create table student( id int primary key auto_increment, class varchar(10), course varchar(16), c_id int unique, foreign key(c_id) references customer(id) on delete cascade on update cascade );

三、记录的相关操作

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查询数据以及。

3.1、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 …;

3.2、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’;

3.3、删除数据DELETE

语法： DELETE FROM 表名 WHERE CONITION; 示例： DELETE FROM mysql.user WHERE password=’’;

3.4、查询数据SELECT

3.4.1、单表查询

1、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

2、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** #准备表和记录 company.emp 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 emp( 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 emp; +--------------+-----------------------+------+-----+---------+----------------+ | 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 emp(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 ===========================================where #多条件查询 select * from emp where id >= 3 and id <= 5; #关键字BETWEEN AND select * from emp where id between 3 and 5; select * from emp where id not between 3 and 5; #关键字IN集合查询 select * from emp where id=3 or id=5 or id=7; select * from emp where id in (3,5,7); select * from emp where id not in (3,5,7); select * from emp where id=3 or id=5 or id=7; #关键字LIKE模糊查询通配符’%’和通配符’_’ select * from emp where name like 'jin%'; select * from emp where name like 'jin___'; #正则查询 select * from emp where name regexp 'n$'; #关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS) mysql> select * from emp where post_comment is not null; Empty set (0.00 sec) mysql> update emp set post_comment='' where id=3; Query OK, 1 row affected (0.07 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from emp where post_comment is not null; +----+---------+------+-----+------------+---------+--------------+---------+--------+-----------+ | id | name | sex | age | hire_date | post | post_comment | salary | office | depart_id | +----+---------+------+-----+------------+---------+--------------+---------+--------+-----------+ | 3 | wupeiqi | male | 81 | 2013-03-05 | teacher | | 8300.00 | 401 | 1 | +----+---------+------+-----+------------+---------+--------------+---------+--------+-----------+ 1 row in set (0.00 sec) mysql> select * from emp where name="丫丫"; select * from emp where name regexp "丫$"; select * from emp where name like "丫_"; select * from emp where name regexp "^程"; select hex(name) from t4 where hex(name) regexp 'e[4-9][0-9a-f]{4}'; ===========================================group by 分完组之后只能看到分组的字段以及聚合的结果 max() min() avg() sum() count() select depart_id,count(id),avg(salary),max(age),min(salary),sum(salary) from emp group by depart_id; # 每个部门都有多少个人 select depart_id,count(id) from emp group by depart_id; # 每个职位男生的平均薪资 select post,avg(salary) from emp where sex="male" group by post; select post, group_concat(name) from emp group by post; select post, group_concat(name) from emp where sex="male" group by post; ===========================================having # having与where本质区别就是在于having是在分组之后发生过滤，可以使用聚合函数 mysql> select max(salary) from emp where max(salary) > 100000; ERROR 1111 (HY000): Invalid use of group function mysql> select max(salary) from emp having max(salary) > 100000; +-------------+ | max(salary) | +-------------+ | 1000000.31 | +-------------+ 1 row in set (0.00 sec) mysql> # 找出来男生平均薪资大于3000的职位 select post,avg(salary) from emp where sex="male" group by post having avg(salary) > 8000; ===========================================order by排序 select * from emp order by salary; select * from emp order by salary desc; select * from emp order by age,id desc; select post,avg(salary) from emp where sex="male" group by post having avg(salary) > 3000; mysql> select post,avg(salary) from emp where sex="male" group by post having avg(salary) > 3000; +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | operation | 16000.043333 | | teacher | 175650.051667 | | 老男孩驻沙河办事处外交大使 | 7300.330000 | +-----------------------------------------+---------------+ 3 rows in set (0.00 sec) mysql> select post,avg(salary) from emp where sex="male" group by post having avg(salary) > 3000 order by avg(salary); +-----------------------------------------+---------------+ | post | avg(salary) | +-----------------------------------------+---------------+ | 老男孩驻沙河办事处外交大使 | 7300.330000 | | operation | 16000.043333 | | teacher | 175650.051667 | +-----------------------------------------+---------------+ 3 rows in set (0.00 sec) mysql> select post,avg(salary) as v from emp where sex="male" group by post having avg(salary) > 3000 order by v; +-----------------------------------------+---------------+ | post | v | +-----------------------------------------+---------------+ | 老男孩驻沙河办事处外交大使 | 7300.330000 | | operation | 16000.043333 | | teacher | 175650.051667 | +-----------------------------------------+---------------+ 3 rows in set (0.00 sec) mysql> select * from emp limit 0,5; select * from emp limit 5,5; select * from emp limit 10,5; select * from emp limit 15,5; select * from emp limit 20,5; ==================使用正则表达式查询============================== SELECT * FROM emp WHERE name REGEXP '^ale'; SELECT * FROM emp WHERE name REGEXP 'on$'; SELECT * FROM emp WHERE name REGEXP 'm{2}'; 小结：对字符串匹配的方式 WHERE name = 'egon'; WHERE name LIKE 'yua%'; WHERE name REGEXP 'on$'

3.4.2、简单查询

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