1：主键约束
可以肯定一张表中的一条记录，经过给某一字段添加约束，就能够使得该字段不重复且不为空。
spa
create table user( -> id int primary key, -> name varchar(20) -> );
咱们使用了主键元素；看下这个表信息。code
MariaDB [helloworld]> describe user; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
咱们能够看到这个Key值是PRI，咱们这时候插入一个元素。rem
nsert into user value(1,'zhangsan'); Query OK, 1 row affected (0.00 sec)
继续插入相同元素就会出现报错table
insert into user value(1,'zhangsan'); ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
咱们添加的时候得更改ID才能进行添加；由于ID有了主键约束；class
insert into user value(2,'zhangsan'); Query OK, 1 row affected (0.00 sec)
查看哈：test
select *from user; +----+----------+ | id | name | +----+----------+ | 1 | zhangsan | | 2 | zhangsan | +----+----------+
注意ID不能为NULL
联合主键：只要联合的主键值加起来不出重复就能够
select
create table user2( id int, -> name vaechar(20). -> password varchar(20), -> primary key(id,name) -> );
insert into user2 values(1,'zhangsan','610722200101555546'); insert into user2 values(2,'zhangsan','610722200101555546');
能够插入的引用
select *from user2; +----+----------+--------------------+ | id | name | password | +----+----------+--------------------+ | 1 | zhangsan | 610722200101555546 | | 2 | zhangsan | 610722200101555546 | +----+----------+--------------------+
2：外键约束
一个是父表，一个是字表
例子：
im
create table classes( id int primary key, name varchar(20));
create table students( id int primary key, name varchar(20), class_id int, foreign key(class_id) references classes(id));
desc classes; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+
desc students; +----------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | NULL | | | name | varchar(20) | YES | | NULL | | | class_id | int(11) | YES | MUL | NULL | | +----------+-------------+------+-----+---------+-------+
外键做用：
主表中没有的数据在父表中是不能够使用的，
主表中的记录被父表引用，是不能够被删除的。
在添加数据或者是删除数据的时候是会被限制的。
3：惟一约束
约束该字段的值不能够重复
handlebars
create table user5( id int, name varchar(20) ); alter table user5 add unique(name);
desc user5; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | UNI | NULL | | +-------+-------------+------+-----+---------+-------+
惟一约束修饰的字段不能够重复
如何删除惟一约束？
alter table user7 drop index name;
desc user5; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+
4：非空约束
create table user7( id int, name varchar(20) not null); Query OK, 0 rows affected (0.01 sec) MariaDB [helloworld]> desc user7; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | NO | | NULL | | +-------+-------------+------+-----+---------+-------+
不传id是能够的
insert into user7(name) values('lisi');
desc user7; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | NO | | NULL | | +-------+-------------+------+-----+---------+-------+
5：自增约束
create table user3( -> id int primary key auto_increment, -> name varchar(20));
自增约束和主键约束搭配在一块儿使用
insert into user3 (name) values('libai');
咱们能够看到，咱们插入时并无写ID号，咱们来看结果
select *from user3; +----+-------+ | id | name | +----+-------+ | 1 | libai | +----+-------+ 1 row in set (0.00 sec)
建立过程当中忘了加主键怎么办
添加主键：
create table user4( id int, name varchar(20));
desc user4; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | PRI | 0 | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
删除主键：
alter table user4 drop primary key;
desc user4; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | 0 | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+
修改主键约束：
alter table user4 modify id int primary key;
6：默认约束
当咱们插入字段值的时候，若是没有传值就会使用默认值。
create table test1( -> id int, -> name varchar(20), -> age int default 10);
desc test1; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | age | int(11) | YES | | 10 | | +-------+-------------+------+-----+---------+-------+
insert into test1(id,name) values(1,'zhangsan');
select *from test1; +------+----------+------+ | id | name | age | +------+----------+------+ | 1 | zhangsan | 10 | +------+----------+------+
===========================================================
总结： 添加约束： 1：建立表的时候添加约束， 2：使用 alter添加…add…， 3：使用 alter…modif…， 4: alter…drop…进行删除。