MySQL约束总结(CONSTRAINT)
约束的含义
一种限制，用于限制表中的数据，为了保证表中的数据的准确和可靠性。MySQL数据库通过约束(constraints)防止无效的数据进入到表中，以保护数据的实体完整性。
约束的分类
在 MySQL 中，主要有六种约束：
1、NOT NULL：非空约束，用于约束该字段的值不能为空。比如姓名、学号等。
2、DEFAULT：默认值约束，用于约束该字段有默认值，约束当数据表中某个字段不输入值时，自动为其添加一个已经设置好的值。比如性别。
3、PRIMARY KEY：主键约束，用于约束该字段的值具有唯一性，至多有一个，可以没有，并且非空。比如学号、员工编号等。
4、UNIQUE：唯一约束，用于约束该字段的值具有唯一性，可以有多个，可以没有，可以为空。比如座位号。
5、CHECK：检查约束，用来检查数据表中，字段值是否有效。比如年龄、性别。
6、FOREIGN KEY：外键约束，外键约束经常和主键约束一起使用，用来确保数据的一致性，用于限制两个表的关系，用于保证该字段的值必须来自于主表的关联列的值。在从表添加外键约束，用于引用主表中某列的值。比如学生表的专业编号，员工表的部门编号，员工表的工种编号。
主要归类为列级约束和表级约束
列级约束：NOT NULL|DEFAULT|PRIMARY KEY|UNIQUE|CHECK表级约束：PRIMARY KEY|UNIQUE|CHECK|FOREIGN KEY
实例
列约束：在字段名和类型后面追加约束类型即可
表约束：在各个列字段的最下面，CONSTRAINT 开头进行约束
CREATE TABLE 表名(
字段名 字段类型 列级约束,
字段名 字段类型,
表级约束
)
创建表时添加约束
-- 列约束
mysql> CREATE TABLE teachers(
-> id INT PRIMARY KEY,
-> stuName VARCHAR(20) NOT NULL UNIQUE,
-> gender CHAR(1) CHECK(gender='W' OR gender='M'),
-> seat INT UNIQUE,
-> age INT DEFAULT 18
-> );
Query OK, 0 rows affected (0.04 sec)
-- 列约束和表约束
mysql> CREATE TABLE students (
-> id INT PRIMARY KEY AUTO_INCREMENT,
-> name VARCHAR(20) NOT NULL,
-> age INT NOT NULL DEFAULT 18,
-> gender CHAR CHECK(gender IN ('W','M')),
-> tid INT,
-> CONSTRAINT fk FOREIGN KEY(tid) REFERENCES teachers(id)
-> );
Query OK, 0 rows affected (0.04 sec)
mysql> show index from teachers;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| teachers | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| teachers | 0 | stuName | 1 | stuName | A | 0 | NULL | NULL | | BTREE | | |
| teachers | 0 | seat | 1 | seat | A | 0 | NULL | NULL | YES | BTREE | | |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
mysql> show index from students;
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | |
| students | 1 | fk | 1 | tid | A | 0 | NULL | NULL | YES | BTREE | | |
+----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
修改表时添加约束
mysql> CREATE TABLE info (
-> id INT,
-> name VARCHAR(20),
-> age INT,
-> gender CHAR,
-> tid INT
-> );
Query OK, 0 rows affected (0.02 sec)
mysql> ALTER TABLE info MODIFY COLUMN id INT PRIMARY KEY; # 添加列主键约束
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info MODIFY COLUMN name VARCHAR(20) NOT NULL; # 添加列非空约束
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info MODIFY COLUMN age INT DEFAULT 18; # 添加列默认约束
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info MODIFY COLUMN tid INT UNIQUE; # 添加列唯一约束
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc info;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | NO | | NULL | |
| age | int(11) | YES | | 18 | |
| gender | char(1) | YES | | NULL | |
| tid | int(11) | YES | UNI | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
mysql> alter table info drop primary key; # 删除列主键约束
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info ADD PRIMARY KEY(id); # 添加“表”主键约束
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info ADD UNIQUE(tid); # 添加“表”唯一约束
Query OK, 0 rows affected, 1 warning (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 1
mysql> ALTER TABLE info ADD CONSTRAINT ue UNIQUE(tid); # 添加“表”唯一约束
Query OK, 0 rows affected, 1 warning (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 1
mysql> desc info;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(11) | NO | PRI | NULL | |
| name | varchar(20) | NO | | NULL | |
| age | int(11) | YES | | 18 | |
| gender | char(1) | YES | | NULL | |
| tid | int(11) | YES | UNI | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.01 sec)
mysql> ALTER TABLE info ADD CONSTRAINT ck CHECK(gender IN ('W','M')); # 添加“表”检查约束
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info ADD CONSTRAINT fy FOREIGN KEY(tid) REFERENCES teachers(id); # 添加“表”外键约束
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
修改表时删除约束
mysql> ALTER TABLE info MODIFY COLUMN name varchar(20) NULL; # 删除列非空约束
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info MODIFY COLUMN age INT(11); # 删除列默认约束
Query OK, 0 rows affected (0.00 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info DROP PRIMARY KEY; # 删除“表”主键约束
Query OK, 0 rows affected (0.07 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info DROP INDEX tid; # 删除“表”唯一约束
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table info \G
*************************** 1. row ***************************
Table: info
Create Table: CREATE TABLE `info` (
`id` int(11) NOT NULL,
`name` varchar(20) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`gender` char(1) DEFAULT NULL,
`tid` int(11) DEFAULT NULL,
UNIQUE KEY `tid_2` (`tid`),
UNIQUE KEY `ue` (`tid`),
CONSTRAINT `fy` FOREIGN KEY (`tid`) REFERENCES `teachers` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312
1 row in set (0.00 sec)
mysql> ALTER TABLE info DROP INDEX ue; # 删除“表”唯一约束
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> ALTER TABLE info DROP FOREIGN KEY fy; # 删除“表”外键约束
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table info \G
*************************** 1. row ***************************
Table: info
Create Table: CREATE TABLE `info` (
`id` int(11) NOT NULL,
`name` varchar(20) DEFAULT NULL,
`age` int(11) DEFAULT NULL,
`gender` char(1) DEFAULT NULL,
`tid` int(11) DEFAULT NULL,
UNIQUE KEY `tid_2` (`tid`)
) ENGINE=InnoDB DEFAULT CHARSET=gb2312
1 row in set (0.00 sec)
mysql> desc info;
+--------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+--------+-------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| name | varchar(20) | YES | | NULL | |
| age | int(11) | YES | | NULL | |
| gender | char(1) | YES | | NULL | |
| tid | int(11) | YES | UNI | NULL | |
+--------+-------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
主键和唯一的区别
| 约束名称 | 保证唯一性 | 是否允许为空 | 一个表中可以有多少个 | 是否允许组合 |
|---|---|---|---|---|
| 主键 | √ | × | 最多有1个，可以没有 | √（不推荐） |
| 唯一 | √ | √ | 可以有多个 | √（不推荐） |