MySQL约束总结(CONSTRAINT)

2022-11-30 14:08:21

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）

码农公寓