mysql中的各种约束条件深入探讨

unsigned   :    无符号
not null   :       不为空
default    :       默认值
unique     :      唯一值,加入唯一索引
(索引相当于字典目录,索引的提出是为了加快速度,一味地乱加索引不会提高查询效率)
primary key:    主键
auto_increment: 自增加一 ，必须设置了主键才能设置该参数
zerofill   :    零填充
foreign key:    外键

create table t3(id int unsigned);
insert into t3 values(-1); error
insert into t3 values(4000000000); success

create table t4(id int not null , name varchar(11));
insert into t4 values(1,"张宇");
insert into t4 values(null,"张宇"); error
insert into t4(name) values("李四"); error

create table t5(id int not null  , name varchar(11) default "沈思雨" );
insert into t5 values(1,null);
insert into t5(id) values(2);

create table t5_2(id int not null  default "1111" , name varchar(11) default "沈思雨" );
insert into t5_2 values(); # 在values里面不写值,默认使用默认值;

create table t6(id int unique , name char(10) default "赵万里" );
insert into t6(id) values(1);
insert into t6(id) values(1); error
insert into t6(id) values(null);
insert into t6(id) values(null); # id变成了多个null

create table t7(id int primary key , name varchar(10) default "赵沈阳");
insert into t7(id) values(1);
insert into t7(id) values(1); error 
insert into t7(id) values(null); error

create table t8(id int unique not null ,  name varchar(10) default "赵沈阳" );

create table t9(id1 int unique not null ,  id2 int primary key );

create table t10(id1 int  primary key  ,  id2 int primary key ); error

create table t11(id int primary key auto_increment , name varchar(255) default "敬文栋");
insert into t11 values(1,"张三");
insert into t11 values(null,"李四");
insert into t11(id) values(null);
# 使用默认值或者自增插入数据
insert into t11 values();

show create table student;
| student | CREATE TABLE `student` (
  `id` int NOT NULL AUTO_INCREMENT,
  `name` varchar(20) NOT NULL,
  `age` int NOT NULL,
  `birthday` date DEFAULT NULL,
  `is_del` tinyint DEFAULT '0',
  `height` decimal(3,2) DEFAULT NULL,
  `cls_id` varchar(6) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `fk_class` (`cls_id`),
  CONSTRAINT `fk_class` FOREIGN KEY (`cls_id`) REFERENCES `class` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=20 DEFAULT CHARSET=utf8 |

create table t12(id int(5) zerofill);
insert into t12 values(1234567);

insert into t12 values(12);