MySQL的四种约束：
※ 主键约束 ：primary key
※ 唯一性约束：unique
※ 外键约束：foreign key
※ 非空约束：not null
※ 默认值约束：default
1. 添加主键约束 PRIMARY KEY
1. 直接在字段后添加：PRIMARY KEY
<字段名> <数据类型> PRIMARY KEY [默认值]
# e.g.
mysql> create table test (id int(5) primary key, name varchar(10));
mysql> desc test ;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(5) | NO | PRI | NULL | |
| name | varchar(10) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2. 建完所有字段后单独指定：
mysql> create table test2
-> (
-> id int(5),
-> name char(10),
-> primary key(id)
-> );
Query OK, 0 rows affected, 1 warning (0.02 sec)
mysql> desc test2;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(5) | NO | PRI | NULL | |
| name | char(10) | YES | | NULL | |
+-------+----------+------+-----+---------+-------+
3. 添加复合主键
将两个字段联合在一起，当成主键
mysql> create table test3
-> (
-> id int(5),
-> name char(10),
-> primary key(id,name)
-> );
Query OK, 0 rows affected, 1 warning (0.02 sec)
mysql> desc test3;
+-------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+-------+
| id | int(5) | NO | PRI | NULL | |
| name | char(10) | NO | PRI | NULL | |
+-------+----------+------+-----+---------+-------+
4. 修改表的主键
1. 表中原来没有主键约束
mysql> alter table student
-> add primary key(id);
Query OK, 0 rows affected (0.05 sec)
mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(5) | NO | PRI | NULL | |
| name | varchar(10) | YES | | NULL | |
| class | varchar(5) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2. 原本表里有主键，需要先删除：
mysql> alter table student drop primary key , add primary key(name);
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc student;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(5) | NO | | NULL | |
| name | varchar(10) | NO | PRI | NULL | |
| class | varchar(5) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+
2. 添加唯一约束 UNIQUE
1. 建表时在字段后面添加
<字段名> <数据类型> UNIQUE
e.g.
mysql> create table test (id int(5) unique);
2. 修改表的唯一约束
ALTER TABLE <数据表名> ADD CONSTRAINT <唯一约束名> UNIQUE(<列名>);
3. 删除表的唯一约束
ALTER TABLE <表名> DROP INDEX <唯一约束名>;
3. 检查约束：CHECK
检查约束（CHECK）可以通过 CREATE TABLE 或 ALTER TABLE 语句实现
CHECK <表达式>
1. 建表时创建
# 创建工资表，添加约束 工资>0,并且<30000
mysql> create table salarys -> ( -> id int(6) primary key, -> salary float, -> name varchar(10), -> check(salary>0 and salary<30000) -> ); Query OK, 0 rows affected, 1 warning (0.03 sec) mysql> desc salarys; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | id | int(6) | NO | PRI | NULL | | | salary | float | YES | | NULL | | | name | varchar(10) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 3 rows in set (0.00 sec) mysql> show create table salarys; +---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | Table | Create Table | +---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ | salarys | CREATE TABLE `salarys` ( `id` int(6) NOT NULL, `salary` float DEFAULT NULL, `name` varchar(10) DEFAULT NULL, PRIMARY KEY (`id`), CONSTRAINT `salarys_chk_1` CHECK (((`salary` > 0) and (`salary` < 30000))) ) ENGINE=InnoDB DEFAULT CHARSET=gb2312 | +---------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec)
2. 修改表的约束
mysql> alter table salarys -> add constraint ck_name -> check(name<>'laowang') -> ; Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table salarysG;
*************************** 1. row ***************************
Table: salarys
Create Table: CREATE TABLE `salarys` (
`id` int(6) NOT NULL,
`salary` float DEFAULT NULL,
`name` varchar(10) DEFAULT NULL,
PRIMARY KEY (`id`),
CONSTRAINT `ck_name` CHECK ((`name` <> _utf8mb3'laowang')),
CONSTRAINT `salarys_chk_1` CHECK (((`salary` > 0) and (`salary` < 30000)))
) ENGINE=InnoDB DEFAULT CHARSET=gb2312
3. 删除约束
ALTER TABLE <数据表名> DROP CONSTRAINT <检查约束名>;
4. 默认约束 DEFAULT
<字段名> <数据类型> DEFAULT <默认值>;
e.g.
1. 创建一个默认字段为‘beijing’的表
mysql> create table test4 -> ( -> location varchar(10) default 'beijing' -> ); Query OK, 0 rows affected (0.03 sec)
2. 修改表的默认约束
ALTER TABLE <数据表名> CHANGE COLUMN <字段名> <数据类型> DEFAULT <默认值>;
3. 删除表的默认约束
ALTER TABLE <数据表名> CHANGE COLUMN <字段名> <字段名> <数据类型> DEFAULT NULL;
5. 非空约束 NOT NULL
1. 建表时创建：
<字段名> <数据类型> NOT NULL;
2. 修改表中约束：
ALTER TABLE <数据表名> CHANGE COLUMN <字段名> <字段名> <数据类型> NOT NULL;
3. 删除约束
ALTER TABLE <数据表名> CHANGE COLUMN <字段名> <字段名> <数据类型> NULL;
6. 外键约束：FOREIGN KEY
[CONSTRAINT <外键名>] FOREIGN KEY 字段名 [，字段名2，…] REFERENCES <主表名> 主键列1 [，主键列2，…]
1. 建表时创建约束：
mysql> create table grade -> ( -> gid int(10), -> record int(10), -> constraint fk_id -> foreign key(gid) references user(id) -> ); Query OK, 0 rows affected, 2 warnings (0.04 sec)
2. 给表添加外键约束：
ALTER TABLE <数据表名> ADD CONSTRAINT <索引名> FOREIGN KEY(<列名>) REFERENCES <主表名> (<列名>);
3. 删除外键约束：
ALTER TABLE <表名> DROP FOREIGN KEY <外键约束名>;