Mysql修饰符notnull有notnull来修饰的话，往表里添加默认值的时候：数值类型的话，添加的是0字符串类型的话，添加的是空如果数据类型是时间戳类型，添加的默认值是当前时间枚举类型，添加的是第一个预先定...SyntaxHighlighter.all();
Mysql修饰符
not null
有not null 来修饰的话，往表里添加默认值的时候：
数值类型的话，添加的是0
字符串类型的话，添加的是空
如果数据类型是时间戳类型，添加的默认值是当前时间
枚举类型，添加的是第一个预先定义的值
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验证举例：
mysql> create table t6(a int not null,b char(10) not null,c timestamp not null,d enum(&＃39;y&＃39;,&＃39;n&＃39;) not null);
mysql> insert into t6 values(); // 添加默认值（就是values后面为空，什么都不写）
mysql> select * from t6;
+---+---+---------------------+---+
| a | b | c | d |
+---+---+---------------------+---+
| 0 | | 2009-12-28 15:42:17 | y |
+---+---+---------------------+---+
----------------------------------------------------------------------------------------------------------------------------
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default
default修饰符为字段指定一个默认值
例子：
> create table t2(id int,name varchar(10),dep varchar(10) default "HR");
> insert into t2 set id=1,name="Lili";
> insert into t2 set id=2,name="Anna";
> insert into t2 set id=3,name="Hebe",dep="MIS";
> select * from t2;
+------+------+------+
| id | name | dep |
+------+------+------+
| 1 | Lili | HR |
| 2 | Anna | HR |
| 3 | Hebe | MIS |
+------+------+------+
auto_increment
auto_increment修饰符只适用于INT字段，表明MySQL应该自动为该字段生成一个数（每次在前一个值得基础上加1）。
MySQL的表只能有一个auto_increment 字段，而且这个字段必须被定义为键。
> create table t3 (id int not null auto_increment primary key,name varchar(10) not null);
> insert into t3(name) values("Anna");
> insert into t3(name) values("Nana");
mysql> select * from t3;
+----+------+
| id | name |
+----+------+
| 1 | Anna |
| 2 | Nana |
+----+------+
-----------------------------------------------------------------------------------
unique
UNI
表示记录不能重复
例子：
> create table user(name char(10),email varchar(20) unique);
> insert into user values("Anna","anna@163.com");
> insert into user values("Nana","anna@163.com");
ERROR 1062 (23000): Duplicate entry &＃39;anna@163.com&＃39; for key 1
unique字段null是允许的
mysql> insert into user values();
mysql> insert into user values();
mysql> select * from user;
+------+--------------+
| name | email |
+------+--------------+
| Anna | anna@163.com |
| NULL | NULL |
| NULL | NULL |
+------+--------------+
删除unique
> alter table user drop index email;
设置unique属性的两种方法：
> create table t10(name char(10),email varchar(20) unique);
> create table t11(id int,name char(10),unique(id));
------------------------------------------------------------------------------------------------------------
索引
MUL
为了加快搜索速度，减少查询时间， MySQL允许我们为一个表的特定字段设置索引
索引的缺点：它占据一定的磁盘空间，而且它影响INSERT，UPDATE和DELETE执行的时间
添加索引
> create table sales(name char(4),price float(4,2),date date,index name_index(name)); // 给字段name添加索引，索引的名称是name_index; 如果没有定义索引名称，则使用字段名称做为索引的名称
查看索引
> show index from sales;
为多个字段添加索引
> create table sales2(name char(4),price float(4,2),date date,index name_index(name),index (price));
> create table sales3(name char(4),price float(4,2),date date,index (name,price));
使用create index命令向已存在的表添加索引
> create index id_index on xueke (id);
删除索引
> drop index id_index on xueke;
------------------------------------------------------------------
主键
primary key
设置主键的几种方法：
> create table pri(id tinyint not null unique,name char(10)); // not null unique
> create table pri(id tinyint primary key,name char(10));
> create table pri(id tinyint,name char(10),primary key(id));
组合主键
> create table firewall(host varchar(20),port smallint(4),access enum(&＃39;deny&＃39;,&＃39;allow&＃39;),primary key(host,port));
> insert into firewall values(&＃39;192.168.10.1&＃39;,21,&＃39;deny&＃39;);
> insert into firewall values(&＃39;192.168.10.1&＃39;,80,&＃39;deny&＃39;);
> insert into firewall values(&＃39;192.168.10.2&＃39;,80,&＃39;deny&＃39;);
> insert into firewall values(&＃39;192.168.10.2&＃39;,80,&＃39;deny&＃39;);
ERROR 1062 (23000): Duplicate entry &＃39;192.168.10.2-80&＃39; for key 1
mysql> select * from firewall;
+--------------+------+--------+
| host | port | access |
+--------------+------+--------+
| 192.168.10.1 | 21 | deny |
| 192.168.10.1 | 80 | deny |
| 192.168.10.2 | 80 | deny |
+--------------+------+--------+
在这种情况下，允许host或者port重复，但不能同时重复
-------------------------------------------------------------------------
外键
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建立外键：
foreign key...references...
> create table score (id int,name char(10) primary key,math int,phy int) type=innodb;
> create table addr (id int primary key,aname char(10) not null,addr char(255),tel int,index (aname),foreign key (aname) references score (name)) type=innodb;
约束：
1. 关系中的所有表必须是InnoDB表
2. 参照的表和字段（必须是primary key,否则会出MySQL - errno:150的错误）是存在 的
3. 组成外键的字段被索引
4. 在外键关系中，字段的数据类型必须相似，这对于大小和符号都必须匹配的整数类型及其重要
mysql> insert into score values(1,"Anna",70,80),(1,"John",80,90),(3,"Lisa",50,60);
mysql> select * from score;
+------+------+------+------+
| id | name | math | phy |
+------+------+------+------+
| 1 | Anna | 70 | 80 |
| 1 | John | 80 | 90 |
| 3 | Lisa | 50 | 60 |
+------+------+------+------+
一旦建立一个外键，MySQL只允许向addr.aname字段输入那些也存在于score.name字段的值
mysql> insert into addr values(1,"Anna","addr...",4334543);
mysql> insert into addr values(2,"Lucy","addr...",8774366);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`db1/addr`, CONSTRAINT `addr_ibfk_1` FOREIGN KEY (`aname`) REFERENCES `score` (`name`))
mysql> select * from addr;
+----+-------+---------+---------+
| id | aname | addr | tel |
+----+-------+---------+---------+
| 1 | Anna | addr... | 4334543 |
+----+-------+---------+---------+
删除外键：
1. 删除有外键的表
> drop table addr;
Query OK, 0 rows affected (0.00 sec)
2. 从表中删除外键
语法：
alter table 表名 drop foreign key 外键名;
show create table 表名;
实验结果 Mysql会报错：
> alter table addr drop foreign key aname;
ERROR 1025 (HY000): Error on rename of &＃39;./db1/addr&＃39; to &＃39;./db1/#sql2-5258-7&＃39; (errno: 152)
on delete cascade子句
cascade删除包含与已删除键值有参照关系的所有记录
建表
> create table score (id int,name char(10) primary key,math int) type=innodb;
> create table addr (id int primary key,aname char(10),addr char(255),tel int,index (aname),foreign key (aname) references score (name) on delete cascade) type=innodb;
插入记录
> insert into score values(1,"Nana",50),(2,"Hebe",70);
> insert into addr values(1,"Nana","addr...",6668787),(2,"Hebe","addr...",8989666);
> select * from score;
+------+------+------+
| id | name | math |
+------+------+------+
| 1 | Nana | 50 |
| 2 | Hebe | 70 |
+------+------+------+
> select * from addr;
+----+-------+---------+---------+
| id | aname | addr | tel |
+----+-------+---------+---------+
| 1 | Nana | addr... | 6668787 |
| 2 | Hebe | addr... | 8989666 |
+----+-------+---------+---------+
删除score表中Hebe的记录，addr表中Hebe的记录自动删除
> delete from score where name="Hebe";
> select * from score;
+------+------+------+
| id | name | math |
+------+------+------+
| 1 | Nana | 50 |
+------+------+------+
> select * from addr;
+----+-------+---------+---------+
| id | aname | addr | tel |
+----+-------+---------+---------+
| 1 | Nana | addr... | 6668787 |
+----+-------+---------+---------+
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