SQL语句建表 约束条件

Posted 2021-07-05 我的紫霞辣辣

约束条件

插入数据的时候可以指定字段

create table t1(id int, name char(16));

insert into t1(name,id)values("nana",1);

select * from t1;
+------+------------------+
| id   | name             |
+------+------------------+
|    1 | nana             |
+------+------------------+

default默认值

create table t2( 
id int, 
name char(16), 
gender enum("male","female","other")default"other" 
); 

desc t2;
+--------+-------------------------------+------+-----+---------+-------+
| Field  | Type                          | Null | Key | Default | Extra |
+--------+-------------------------------+------+-----+---------+-------+
| id     | int(11)                       | YES  |     | NULL    |       |
| name   | char(16)                      | YES  |     | NULL    |       |
| gender | enum('male','female','other') | YES  |     | other   |       |
+--------+-------------------------------+------+-----+---------+-------+

insert into t2 values(1,"nana","female");
insert into t2(id,name) values(2,"lala");

select * from t2;
+------+------------------+--------+
| id   | name             | gender |
+------+------------------+--------+
|    1 | nana             | female |
|    2 | lala             | other  |
+------+------------------+--------+

unique唯一

单列唯一

create table t3(
id int unique,
name char(16)
);

# id重复,数据存储失败
insert into t3 values(1,"nana"),(1,"dada");
ERROR 1062 (23000): Duplicate entry '1' for key 'id'

insert into t3 values(1,"nana"),(2,"dada");   
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

desc t3;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | YES  | UNI | NULL    |       |
| name  | char(16) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

select * from t3;
+------+------------------+
| id   | name             |
+------+------------------+
|    1 | nana             |
|    2 | dada             |
+------+------------------+

联合唯一

create table t4( 
id int, ip char(16), 
port int, 
unique(ip,port) 
);

insert into t4 values(1,"127.0.0.1",8080);
insert into t4 values(1,"127.0.0.1",8081);
insert into t4 values(1,"127.0.0.2",8080);

# 指定联合唯一的字段重复了,无法存储数据
insert into t4 values(1,"127.0.0.1",8080);
ERROR 1062 (23000): Duplicate entry '127.0.0.1       -8080' for key 'ip'

desc t4;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | YES  |     | NULL    |       |
| ip    | char(16) | YES  | MUL | NULL    |       |
| port  | int(11)  | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

select * from t4;
+------+------------------+------+
| id   | ip               | port |
+------+------------------+------+
|    1 | 127.0.0.1        | 8080 |
|    2 | 127.0.0.1        | 8081 |
|    2 | 127.0.0.2        | 8080 |
+------+------------------+------+

primary key主键

单单从约束效果上来看primary key 等价于not null + unique，非空且唯一！！！

create table t5(id int primary key);

# 报错,插入的数据不能为null
insert into t5 values(null);
ERROR 1048 (23000): Column 'id' cannot be null

# 报错,插入的数据不能重复
insert into t5 values(1),(1);
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

# 非空且唯一
insert into t5 values(1),(2);
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

desc t5;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+

select * from t5;
+----+
| id |
+----+
|  1 |
|  2 |
+----+

primary key的属性

primary key它除了有约束效果之外，它还是Innodb存储引擎组织数据的依据，Innodb存储引擎在创建表的是后必须要有primary key。因为它类似于书的目录，能够帮助数据库提升查询效率并且也是建表的依据。

一张表中有且只有一个主键，如果你没有设置主键，那么数据库会默认从上往下搜索，直到遇到一个非空且唯一的字段将它自动升级为主键！！！主键能够帮助数据库提升查询效率！！！同时也是建表的依据(主键必须存在)

mysql> create table t6(
id int,
name char(16),
age int not null unique,
addr char(32) not null unique
);

desc t6;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | YES  |     | NULL    |       |
| name  | char(16) | YES  |     | NULL    |       |
| age   | int(11)  | NO   | PRI | NULL    |       |
| addr  | char(32) | NO   | UNI | NULL    |       |
+-------+----------+------+-----+---------+-------+

如果表中没有主键也没有其他任何的非空且唯一字段，那么Innodb会采用自己内部提供的一个隐藏字段为主键，隐藏意味着你无法使用它，就无法提供查询速度。一张表中通常都应该有一个主键字段 并且通常将id字段作为主键。

单个字段主键

mysql> create table t7( id int primary key, name char(16) );

mysql> desc t7;
+-------+----------+------+-----+---------+-------+
| Field | Type     | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id    | int(11)  | NO   | PRI | NULL    |       |
| name  | char(16) | YES  |     | NULL    |       |
+-------+----------+------+-----+---------+-------+

联合主键

联合主键只是多个字段联合起来作为表的主键，本质上还是一个主键。

create table t8(
id int,
port int,
primary key(id,port)
);

mysql> desc t8;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | 0       |       |
| port  | int(11) | NO   | PRI | 0       |       |
+-------+---------+------+-----+---------+-------+

我们以后在创建表的时候id字段一定要加primary key(数据的唯一标识)

auto_increment自增

auto_increment自增：自动增加数据的编号

create table t9(
id int primary key auto_increment,
name char(16)
);

insert into t9(name) values("nana"),("dada");

select * from t9;
+----+------------------+
| id | name             |
+----+------------------+
|  1 | nana             |
|  2 | dada             |
+----+------------------+

注意auto_increment通常都是加在primary key主键上,不能给普通字段加!!!


补充说明:
# 在删除表中数据的时候，主键的自增不会停止		
delete from t9;

insert into t9(name) values("lala");

# id默认是从4开始的
select * from t9;
+----+------------------+
| id | name             |
+----+------------------+
|  4 | lala             |
+----+------------------+

=======================================================================================================================
# 清空表数据并重置主键
truncate t9;

mysql> insert into t9(name) values("lala");

# id是重置为0开始的
mysql> select * from t9;
+----+------------------+
| id | name             |
+----+------------------+
|  1 | lala             |
+----+------------------+