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...)
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
例子
create table t15( id int(11) unsigned zerofill ); mysql> create table t15( -> id int(11) unsigned zerofill -> ); Query OK, 0 rows affected (0.38 sec) mysql> desc t15; +-------+---------------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------------------------+------+-----+---------+-------+ | id | int(11) unsigned zerofill | YES | | NULL | | +-------+---------------------------+------+-----+---------+-------+ 1 row in set (0.01 sec) int约束是整形，11表示宽度，unsigned 表示无符号，zerofill表示不够用0填充。 create table t16( id int(11), name char(6), sex enum(‘male‘,‘female‘) not null default ‘male‘ ); mysql> create table t16( -> id int(11), -> name char(6), -> sex enum(‘male‘,‘female‘) not null default ‘male‘ -> ); Query OK, 0 rows affected (0.42 sec) mysql> desc t16; +-------+-----------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-----------------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | char(6) | YES | | NULL | | | sex | enum(‘male‘,‘female‘) | NO | | male | | +-------+-----------------------+------+-----+---------+-------+ 3 rows in set (0.00 sec) # enum表示枚举，not null 表示不为空，default ‘male‘ 表示sex不传入的话默认值是male 例子 mysql> insert into t16(id,name) values(1,‘cmz‘); # sex没有传入，默认为male Query OK, 1 row affected (0.36 sec) mysql> desc t16; +-------+-----------------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-----------------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | char(6) | YES | | NULL | | | sex | enum(‘male‘,‘female‘) | NO | | male | | +-------+-----------------------+------+-----+---------+-------+ 3 rows in set (0.00 sec) mysql> select * from t16; +------+------+------+ | id | name | sex | +------+------+------+ | 1 | cmz | male | +------+------+------+ 1 row in set (0.00 sec)
==================not null==================== mysql> create table t1(id int); #id字段默认可以插入空 mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t1 values(); #可以插入空 mysql> create table t2(id int not null); #设置字段id不为空 mysql> desc t2; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | +-------+---------+------+-----+---------+-------+ mysql> insert into t2 values(); #不能插入空 ERROR 1364 (HY000): Field ‘id‘ doesn‘t have a default value ==================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> alter table t3 modify id int not null default 1; ==================综合练习==================== mysql> create table student( -> name varchar(20) not null, -> age int(3) unsigned not null default 18, -> sex enum(‘male‘,‘female‘) default ‘male‘, -> hobby set(‘play‘,‘study‘,‘read‘,‘music‘) default ‘play,music‘ -> ); mysql> desc student; +-------+------------------------------------+------+-----+------------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+------------------------------------+------+-----+------------+-------+ | name | varchar(20) | NO | | NULL | | | age | int(3) unsigned | NO | | 18 | | | sex | enum(‘male‘,‘female‘) | YES | | male | | | hobby | set(‘play‘,‘study‘,‘read‘,‘music‘) | YES | | play,music | | +-------+------------------------------------+------+-----+------------+-------+ mysql> insert into student(name) values(‘egon‘); mysql> select * from student; +------+-----+------+------------+ | name | age | sex | hobby | +------+-----+------+------------+ | egon | 18 | male | play,music | +------+-----+------+------------+ 验证
unique
#-------------------------------------------unique key # 单列唯一 # 方式1 create table department( id int, name char(10) unique ); # 方式2 create table department( id int, name char(10), unique(name) ); insert into department values(1,‘IT‘),(2,‘sales‘); insert into department values(3,‘IT‘); # 会报错 mysql> create table department( -> id int, -> name char(10) unique -> ); Query OK, 0 rows affected (0.04 sec) mysql> insert into department values(1,‘IT‘),(2,‘sales‘); Query OK, 2 rows affected (0.04 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> desc department; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | char(10) | YES | UNI | NULL | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> select * from department; +------+-------+ | id | name | +------+-------+ | 1 | IT | | 2 | sales | +------+-------+ 2 rows in set (0.00 sec) mysql> insert into department values(3,‘IT‘); ERROR 1062 (23000): Duplicate entry ‘IT‘ for key ‘name‘ 其他要是确定唯一性，就添加unique即可。 # 联合唯一 create table services( id int, ip char(15), port int, unique(id), unique(ip,port) ); insert into services values (1,‘192.168.1.100‘,80), (2,‘192.168.1.100‘,8080), (3,‘192.168.1.110‘,80); insert into services values (1,‘192.168.1.100‘,80); #会报错 mysql> create table services( -> id int, -> ip char(15), -> port int, -> unique(id), -> unique(ip,port) -> ); Query OK, 0 rows affected (0.41 sec) mysql> desc services; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | YES | UNI | NULL | | | ip | char(15) | YES | MUL | NULL | | | port | int(11) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 3 rows in set (0.01 sec) mysql> insert into services values -> (1,‘192.168.1.100‘,80), -> (2,‘192.168.1.100‘,8080), -> (3,‘192.168.1.110‘,80); Query OK, 3 rows affected (0.37 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from services; +------+---------------+------+ | id | ip | port | +------+---------------+------+ | 1 | 192.168.1.100 | 80 | | 2 | 192.168.1.100 | 8080 | | 3 | 192.168.1.110 | 80 | +------+---------------+------+ 3 rows in set (0.00 sec) mysql> insert into services values (1,‘192.168.1.100‘,80); #会报错 ERROR 1062 (23000): Duplicate entry ‘1‘ for key ‘id‘
============设置唯一约束 UNIQUE=============== 方法一： create table department1( id int, name varchar(20) unique, comment varchar(100) ); 方法二： create table department2( id int, name varchar(20), comment varchar(100), constraint uk_name unique(name) ); mysql> insert into department1 values(1,‘IT‘,‘技术‘); Query OK, 1 row affected (0.00 sec) mysql> insert into department1 values(1,‘IT‘,‘技术‘); ERROR 1062 (23000): Duplicate entry ‘IT‘ for key ‘name‘ mysql> create table t1(id int not null unique); Query OK, 0 rows affected (0.02 sec) mysql> desc t1; +-------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+---------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | +-------+---------+------+-----+---------+-------+ row in set (0.00 sec) not null+unique的化学反应 create table service( id int primary key auto_increment, name varchar(20), host varchar(15) not null, port int not null, unique(host,port) #联合唯一 ); mysql> insert into service values -> (1,‘nginx‘,‘192.168.0.10‘,80), -> (2,‘haproxy‘,‘192.168.0.20‘,80), -> (3,‘mysql‘,‘192.168.0.30‘,3306) -> ; Query OK, 3 rows affected (0.01 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> insert into service(name,host,port) values(‘nginx‘,‘192.168.0.10‘,80); ERROR 1062 (23000): Duplicate entry ‘192.168.0.10-80‘ for key ‘host‘ 联合唯一
primary key
primary key字段的值不为空且唯一
一个表中可以：
1. 单列做主键
2. 多列做主键（复合主键）
3. 一个表内只能有一个主键primary key
# primary key 约束 not null unique 存储引擎(innodb)，对于innodb存储引擎来说，一个表内必须有一个主键。 # 单列主键 create table t17( id int primary key, name char(16) ); insert into t17 values (1,‘cmz‘), (2,‘leco‘), (3,‘nanjing‘); insert into t17 values (1,‘nanjing‘); # 重复了 mysql> create table t17( -> id int primary key, -> name char(16) -> ); Query OK, 0 rows affected (0.48 sec) mysql> desc t17; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | char(16) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.01 sec) mysql> insert into t17 values (1,‘nanjing‘); ERROR 1062 (23000): Duplicate entry ‘1‘ for key ‘PRIMARY‘ # mysql建表的时候要是没有指定主键，mysql会搜整个表找一个不为空，且唯一的作为主键 要是找不到，就会默认使用隐藏主键。 create table t18( id int not null unique, name char(16) ); mysql> create table t18( -> id int not null unique, -> name char(16) -> ); Query OK, 0 rows affected (0.40 sec) mysql> desc t18; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | # 可以看出作为了主键 | name | char(16) | YES | | NULL | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.00 sec) 通常一个表中有一个id字段作为主键。 # 符合主键 多个字段联合在一起作为主键 create table t19( ip char(15), port int, primary key(ip,port) ); create table loocha( ip varchar(15), port char(5), service_name varchar(10) not null, primary key(ip,port) ); mysql> create table t19( -> ip char(15), -> port int, -> primary key(ip,port) -> ); Query OK, 0 rows affected (0.39 sec) mysql> desc t19; +-------+----------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+-------+ | ip | char(15) | NO | PRI | NULL | | | port | int(11) | NO | PRI | NULL | | +-------+----------+------+-----+---------+-------+ 2 rows in set (0.00 sec) mysql> create table loocha( -> ip varchar(15), -> port char(5), -> service_name varchar(10) not null, -> primary key(ip,port) -> ); Query OK, 0 rows affected (0.03 sec) mysql> desc loocha -> ; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | ip | varchar(15) | NO | PRI | NULL | | | port | char(5) | NO | PRI | NULL | | | service_name | varchar(10) | NO | | NULL | | +--------------+-------------+------+-----+---------+-------+ 3 rows in set (0.01 sec) insert into t19 values (‘192.168.1.110‘,80); insert into t19 values (‘192.168.1.110‘,80); # 报错 mysql> insert into t19 values (‘192.168.1.110‘,80); Query OK, 1 row affected (0.01 sec) mysql> insert into t19 values (‘192.168.1.110‘,80); ERROR 1062 (23000): Duplicate entry ‘192.168.1.110-80‘ for key ‘PRIMARY‘ mysql> select * from t19; +---------------+------+ | ip | port | +---------------+------+ | 192.168.1.110 | 80 | +---------------+------+ 1 row in set (0.00 sec)
============单列做主键=============== #方法一：not null+unique create table department1( id int not null unique, #主键 name varchar(20) not null unique, comment varchar(100) ); mysql> desc department1; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | NO | UNI | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) #方法二：在某一个字段后用primary key create table department2( id int primary key, #主键 name varchar(20), comment varchar(100) ); mysql> desc department2; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.00 sec) #方法三：在所有字段后单独定义primary key create table department3( id int, name varchar(20), comment varchar(100), constraint pk_name primary key(id); #创建主键并为其命名pk_name mysql> desc department3; +---------+--------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+--------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | comment | varchar(100) | YES | | NULL | | +---------+--------------+------+-----+---------+-------+ rows in set (0.01 sec) 单列主键 ==================多列做主键================ create table service( ip varchar(15), port char(5), service_name varchar(10) not null, primary key(ip,port) ); mysql> desc service; +--------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+-------+ | ip | varchar(15) | NO | PRI | NULL | | | port | char(5) | NO | PRI | NULL | | | service_name | varchar(10) | NO | | NULL | | +--------------+-------------+------+-----+---------+-------+ rows in set (0.00 sec) mysql> insert into service values -> (‘172.16.45.10‘,‘3306‘,‘mysqld‘), -> (‘172.16.45.11‘,‘3306‘,‘mariadb‘) -> ; Query OK, 2 rows affected (0.00 sec) Records: 2 Duplicates: 0 Warnings: 0 mysql> insert into service values (‘172.16.45.10‘,‘3306‘,‘nginx‘); ERROR 1062 (23000): Duplicate entry ‘172.16.45.10-3306‘ for key ‘PRIMARY‘ 多列主键
auto_increment
# 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 # primary key or unique key #----------------------------------------------------------- # #不指定id，则自动增长 create table t20( id int primary key auto_increment, name char(16) ); mysql> create table t20( -> id int primary key auto_increment, -> name char(16) -> ); Query OK, 0 rows affected (0.02 sec) mysql> desc t20; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | char(16) | YES | | NULL | | +-------+----------+------+-----+---------+----------------+ 2 rows in set (0.02 sec) insert into t20(name) values(‘cmz1‘),(‘leco‘),(‘loocha‘); mysql> insert into t20(name) values(‘cmz1‘),(‘leco‘),(‘loocha‘); Query OK, 3 rows affected (0.36 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from t20; +----+--------+ | id | name | +----+--------+ | 1 | cmz1 | | 2 | leco | | 3 | loocha | +----+--------+ 3 rows in set (0.00 sec) insert into t20(id,name) values(7,‘keke‘); # 可以手动插入primary key只要保证主键唯一即可，以后在插入就在该基础上自增加 mysql> insert into t20(id,name) values(7,‘cmz1‘); Query OK, 1 row affected (0.00 sec) mysql> select * from t20; +----+--------+ | id | name | +----+--------+ | 1 | cmz1 | | 2 | leco | | 3 | loocha | | 7 | cmz1 | +----+--------+ 4 rows in set (0.01 sec) 自增长步长默认为1 show variables like ‘auto_inc%‘; mysql> show variables like ‘auto_inc%‘; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 1 | # 步长 默认为1 | auto_increment_offset | 1 | # 起始偏移量 默认为1 +--------------------------+-------+ 2 rows in set (0.08 sec) # 设置步长 # session 有效 set session auto_increment_increment=5; # 全局 set global auto_increment_increment=5; # 设置起始偏移量 set global auto_increment_offset=3; 强调起始偏移量<=步长 #例子设置不步长和起始偏移量 mysql> set global auto_increment_offset=3; Query OK, 0 rows affected (0.00 sec) mysql> set global auto_increment_increment=5; Query OK, 0 rows affected (0.00 sec) 重新登录。 mysql> show variables like ‘auto_inc%‘; +--------------------------+-------+ | Variable_name | Value | +--------------------------+-------+ | auto_increment_increment | 5 | | auto_increment_offset | 3 | +--------------------------+-------+ 2 rows in set (0.01 sec) create table t21( id int primary key auto_increment, name char(16) ); mysql> create table t21( -> id int primary key auto_increment, -> name char(16) -> ); Query OK, 0 rows affected (0.40 sec) 插入数据 insert into t21(name) values (‘cmz‘),(‘leco‘),(‘loocha‘); mysql> insert into t21(name) values -> (‘cmz‘),(‘leco‘),(‘loocha‘); Query OK, 3 rows affected (0.02 sec) Records: 3 Duplicates: 0 Warnings: 0 mysql> select * from t21; +----+--------+ | id | name | +----+--------+ | 3 | cmz | | 8 | leco | | 13 | loocha | +----+--------+ 3 rows in set (0.00 sec) # 清空表 delete from t20; mysql> delete from t20; Query OK, 4 rows affected (0.01 sec) mysql> select * from t20; Empty set (0.00 sec) mysql> show create table t20; +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | t20 | CREATE TABLE `t20` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` char(16) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=latin1 | +-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) # 虽然清空了表，但是主键没删除 mysql> insert into t20(name) values(‘cccc‘); Query OK, 1 row affected (0.01 sec) mysql> select * from t20; +----+------+ | id | name | +----+------+ | 8 | cccc | +----+------+ 1 row in set (0.00 sec) #truncate 清空表，主键也删除 mysql> truncate t20; Query OK, 0 rows affected (0.03 sec) mysql> show create table t20; +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ | t20 | CREATE TABLE `t20` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` char(16) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=latin1 | +-------+----------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) mysql> insert into t20(name) values(‘cccc‘); Query OK, 1 row affected (0.01 sec) mysql> select * from t20; +----+------+ | id | name | +----+------+ | 1 | cccc | +----+------+ 1 row in set (0.00 sec)
#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#不指定id，则自动增长 create table student( id int primary key auto_increment, name varchar(20), sex enum(‘male‘,‘female‘) default ‘male‘ ); 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 | | +-------+-----------------------+------+-----+---------+----------------+ mysql> insert into student(name) values -> (‘egon‘), -> (‘alex‘) -> ; mysql> select * from student; +----+------+------+ | id | name | sex | +----+------+------+ | 1 | egon | male | | 2 | alex | male | +----+------+------+ #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 | +----+------+------+ row in set (0.00 sec) #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 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 | +----+-------+------+ 步长increment与起始偏移量offset:auto_increment_increment,auto_increment_offset
auto_increment时增加新内容
清空表：
delete from t1; #如果有自增id，新增的数据，仍然是以删除前的最后一样作为起始。
truncate table t1;数据量大，删除速度比上一条快，且直接从零开始，