06.约束

约束

保证数据的完整性和一致性
表级约束和列级约束
约束类型：
NOT NULL（非空约束）
PRIMARY KEY（主键约束）
UNIQUE KEY（唯一约束）
DEFAULT（默认约束）
FOREIGN KEY（外键约束）

非空约束(NOT NULL)

创建表 其中 not_null字段不许为空
mysql> CREATE TABLE null_test(
    -> not_null VARCHAR(20) NOT NULL,
    -> can_null VARCHAR(20) NULL,
    -> null_ VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.39 sec)
查看表结构
mysql> DESC null_test;
+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| not_null | varchar(20) | NO   |     | NULL    |       |
| can_null | varchar(20) | YES  |     | NULL    |       |
| null_    | varchar(20) | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)
后两个为NULL，插入正常
mysql> INSERT null_test VALUES('tom',NULL,NULL);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT*FROM null_test;
+----------+----------+-------+
| not_null | can_null | null_ |
+----------+----------+-------+
| tom      | NULL     | NULL  |
+----------+----------+-------+
1 row in set (0.00 sec)
第一个为NULL，插入异常
mysql> INSERT null_test VALUES(NULL,'jojo',NULL);
ERROR 1048 (23000): Column 'not_null' cannot be null

主键约束（PRIMARY KEY）

主键（PRIMARY KEY）
主键约束
每张表只能有一个主键
主键字段值不能相同
主键自动NOT NULL
自增长依附于主键，但主键不一定需要自增长
自增长错误
mysql> CREATE TABLE unique_id(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT,
    -> username VARCHAR(20)
    -> );
ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key
自增长的字段必须为主键
mysql> CREATE TABLE unique_id(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> DESC unique_id;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)          | YES  |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)
插入数据，主键字段值自增长
mysql> INSERT unique_id(username) VALUES(one);
ERROR 1054 (42S22): Unknown column 'one' in 'field list'
mysql> INSERT unique_id(username) VALUES('one');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT unique_id(username) VALUES('two');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT unique_id(username) VALUES('three');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT*FROM unique_id;
+----+----------+
| id | username |
+----+----------+
|  1 | one      |
|  2 | two      |
|  3 | three    |
+----+----------+
3 rows in set (0.00 sec)
id设为了主键，当001存在，再插入001，就会报错
mysql> CREATE TABLE key_test(
    -> id SMALLINT UNSIGNED PRIMARY KEY,
    -> username VARCHAR(20)
    -> );
Query OK, 0 rows affected (0.39 sec)

mysql> INSERT key_test VALUES(001,'dog');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT key_test VALUES(002,'dog');
Query OK, 1 row affected (0.01 sec)

mysql> INSERT key_test VALUES(001,'cat');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

唯一约束（UNIQUE KEY）

可为NULL
一表可存在多个
username 设置了唯一约束，当重复时，插入异常
mysql> CREATE TABLE unique_test(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(20) UNIQUE KEY,
    -> age TINYINT UNSIGNED
    -> );
Query OK, 0 rows affected (0.06 sec)

mysql> DESC unique_test;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(20)          | YES  | UNI | NULL    |                |
| age      | tinyint(3) unsigned  | YES  |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+
3 rows in set (0.01 sec)

mysql> INSERT unique_test(username,age) VALUES('dog',22);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT unique_test(username,age) VALUES('cat',22);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT unique_test(username,age) VALUES('dog',12);
ERROR 1062 (23000): Duplicate entry 'dog' for key 'username'

外键约束(FOREIGN KEY)

父表和子表必须使用相同储存引擎
数据表的储存引擎只能是InnoDB
外键列和参照列必须由相似的数据类型
外键列和参照列必须创建索引
创建省份表

mysql> CREATE TABLE province(
    ->    id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    ->    p_name VARCHAR(20) NOT NULL
    ->    );
Query OK, 0 rows affected (0.03 sec)

mysql> DESC province;
+--------+----------------------+------+-----+---------+----------------+
| Field  | Type                 | Null | Key | Default | Extra          |
+--------+----------------------+------+-----+---------+----------------+
| id     | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| p_name | varchar(20)          | NO   |     | NULL    |                |
+--------+----------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
查询建表语句，可见使用了InnoDB
mysql> SHOW CREATE TABLE province;
+----------+-------------------------------------------
| Table    | Create Table
+----------+-------------------------------------------
| province | CREATE TABLE `province` (
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `p_name` varchar(20) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+----------+-------------------------------------------
1 row in set (0.00 sec)
类型不统一，不能创建外键约束表
mysql> CREATE TABLE users(
    ->    id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    ->    username VARCHAR(20) NOT NULL,
    ->    p_id BIGINT,
    ->    FOREIGN KEY(p_id)REFERENCES province(id)
    ->    );
ERROR 1215 (HY000): Cannot add foreign key constraint
统一后，创建成功
mysql> CREATE TABLE users(
    ->    id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    ->    username VARCHAR(20) NOT NULL,
    ->    p_id SMALLINT UNSIGNED,
    ->    FOREIGN KEY(p_id)REFERENCES province(id)
    ->    );
Query OK, 0 rows affected (0.39 sec)
查看province表的索引（G以网格形式，不加则以表的形式，太长）
mysql> SHOW INDEXES FROM province G;
*************************** 1. row ***************************
        Table: province
   Non_unique: 0
     Key_name: PRIMARY
 Seq_in_index: 1
  Column_name: id
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
1 row in set (0.00 sec)
查看users表的索引
*************************** 1. row ***************************
        Table: users
   Non_unique: 0
     Key_name: PRIMARY
 Seq_in_index: 1
  Column_name: id
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null:
   Index_type: BTREE
      Comment:
Index_comment:
*************************** 2. row ***************************
        Table: users
   Non_unique: 1
     Key_name: p_id
 Seq_in_index: 1
  Column_name: p_id
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment:
Index_comment:
2 rows in set (0.01 sec)
外键约束参照操作
CASCADE:从父表删除或更新且自动删除或更新子表中匹配的行
SET NULL:从父表删除或更新行，并设置子表中外键列为NULL。（需子表列没有指定NOT NULL）
RESTRICT：拒绝对父表的删除或更新操作
NO ACTION：标准SQL关键字
CASCADE测试：
建表user1
mysql> CREATE TABLE user1(
    ->    id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    ->    username VARCHAR(20) NOT NULL,
    ->    p_id SMALLINT UNSIGNED,
    ->    FOREIGN KEY(p_id)REFERENCES province(id) ON DELETE CASCADE
    ->    );
Query OK, 0 rows affected (0.06 sec)

插入记录：必须在父表插入数据，插入三个省份

mysql> INSERT province(p_name) VALUES('福建省');
Query OK, 1 row affected (0.00 sec)

mysql> INSERT province(p_name) VALUES('山东省');
Query OK, 1 row affected (0.00 sec)

mysql> SELECT*FROM province;
+----+-----------+
| id | p_name    |
+----+-----------+
|  1 | 安徽省    |
|  2 | 福建省    |
|  3 | 山东省    |
+----+-----------+
3 rows in set (0.00 sec)
在子表中插入记录：（若超出父表范围，会插入失败（也会占一个Id））
mysql> INSERT user1(username,p_id) VALUES('张风',1);
Query OK, 1 row affected (0.00 sec)
mysql> INSERT user1(username,p_id) VALUES('toly',10);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails
 (`zoom`.`user1`, CONSTRAINT `user1_ibfk_1` FOREIGN KEY (`p_id`)
 REFERENCES `province` (`id`) ON DELETE CASCADE)
 mysql> INSERT user1(username,p_id) VALUES('捷特',2);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT user1(username,p_id) VALUES('龙少',1);
Query OK, 1 row affected (0.00 sec)
mysql>  SELECT*FROM user1;
+----+----------+------+
| id | username | p_id |
+----+----------+------+
|  1 | 张风     |    1 |
|  3 | 捷特     |    2 |
|  4 | 龙少     |    1 |
+----+----------+------+
3 rows in set (0.00 sec)
删除province 表中id是2和3的行，结果子表相应行也被删除
mysql> DELETE FROM province WHERE id=2;
Query OK, 1 row affected (0.01 sec)
mysql> DELETE FROM province WHERE id=3;
Query OK, 1 row affected (0.00 sec)
mysql>  SELECT*FROM province;
+----+-----------+
| id | p_name    |
+----+-----------+
|  1 | 安徽省    |
+----+-----------+
1 row in set (0.00 sec)
mysql>  SELECT*FROM user1;
+----+----------+------+
| id | username | p_id |
+----+----------+------+
|  1 | 张风     |    1 |
|  4 | 龙少     |    1 |
+----+----------+------+
2 rows in set (0.00 sec)
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原文地址：https://www.cnblogs.com/toly-top/p/9782028.html