表的约束

1.约束是一种限制，它通过对表的行或列的数据做出限制，来确保表的数据的完整性、唯一性。

2.按照约束的数目划分——分为表级约束和列级约束。ps：如果约束针对一个字段我们称为列级约束，针对两个或两个以上我们称为表级约束
3.按照功能划分——约束分为：
- 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）

列级约束即可在列定义时声明，也可以在列定义后声明，表级约束只能在列定义后声明。
NOT NULL，DEFAULT只有列级约束。
check 约束不起作用。

---

其他约束看这里

FOREIGN KEY

保持数据的一致性，完整性。
实现一对一或一对多关系。（把数据库称为关系型数据库的原因）

外键约束的条件

1.父表（子表参照的表）和子表（具有外键列的表）必须使用相同的存储引擎，而且禁止使用临时表。
2.数据表的存储引擎只能为InnoDB。
3.外键列和参照列必须具有相似的数据类型。其中数字的长度或是否有符号必须相同；而字符的长度则可以不同。
4.外键列和参照列必须创建索引。如果外键列不存在索引的话，MySQL将自动创建索引。

编辑数据表的默认存储引擎

MySQL配置文件
default-storage-engine = INNODB

查看存储引擎

mysql> \q

mysql> use ti
Database changed

mysql> CREATE TABLE provinces(
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> pname VARCHAR(20) NOT NULL
    -> );
Query OK, 0 rows affected (0.23 sec)

mysql> SHOW CREATE TABLE provinces;
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table     | Create Table                                                                                                                                                              |
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| provinces | CREATE TABLE `provinces` (
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `pname` varchar(20) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-----------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

创建外键

FOREIGN KEY(pid) REFERENCES provinces(id)
foreign references

//父表
 mysql> CREATE TABLE provinces(
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> pname VARCHAR(20) NOT NULL
    -> );
Query OK, 0 rows affected (0.23 sec)

//子表
mysql> CREATE TABLE users(
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> username VARCHAR(10) NOT NULL,
    -> pid SMALLINT,
    -> FOREIGN KEY(pid) REFERENCES provinces(id)
    -> );
ERROR 1215 (HY000): Cannot add foreign key constraint
//其中数字的长度或是否有符号必须相同

mysql> CREATE TABLE users(
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> username VARCHAR(10) NOT NULL,
    -> pid SMALLINT UNSIGNED,
    -> FOREIGN KEY(pid) REFERENCES provinces(id)
    -> );
Query OK, 0 rows affected (0.19 sec)

查看索引

show INDEXES FROM tab_name;
show INDEXES FROM tab_name\g; 以网格的形式

mysql> show INDEXES FROM provinces;
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table     | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| provinces |          0 | PRIMARY  |            1 | id          | A         |           0 |     NULL | NULL   |      | BTREE      |         |               |
+-----------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)


// 以网格的形式
mysql> show INDEXES FROM provinces\G;
*************************** 1. row ***************************
        Table: provinces
   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)

ERROR:
No query specified

子表的索引子

表有两个索引

mysql> show INDEXES FROM users\G;
*************************** 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: pid
 Seq_in_index: 1
  Column_name: pid
    Collation: A
  Cardinality: 0
     Sub_part: NULL
       Packed: NULL
         Null: YES
   Index_type: BTREE
      Comment:
Index_comment:
2 rows in set (0.00 sec)

ERROR:
No query specified

mysql> SHOW CREATE TABLE users;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                            |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| users | CREATE TABLE `users` (
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `username` varchar(10) NOT NULL,
  `pid` smallint(5) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `pid` (`pid`),
  CONSTRAINT `users_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `provinces` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

外键约束参照的操作 restrict

• CASCADE（cascade）：从父表删除或跟新且自动删除或更新表中匹配的行
• SET NULL: 从父表中删除或更新行，并设置子表中的外键列为null。如果使用该选项，必须保证子表列没有指定NOT NULL
• RESTRICT(restrict):拒绝对父表的删除或跟新操作。
• NO ACTION : 标准SQL的关键字，在MySQL中与RESTRUCT相同。

mysql> CREATE TABLE user1(
    -> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
    -> username VARCHAR(10) NOT NULL,
    -> pid SMALLINT UNSIGNED,
    -> FOREIGN KEY(pid) REFERENCES provinces(id) ON DELETE CASCADE
    -> );
Query OK, 0 rows affected (0.20 sec)

mysql> SHOW CREATE TABLE USER1;
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                                              |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| USER1 | CREATE TABLE `user1` (
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `username` varchar(10) NOT NULL,
  `pid` smallint(5) unsigned DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `pid` (`pid`),
  CONSTRAINT `user1_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `provinces` (`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

做插入记录操作时要先在父表中插入再在子表中插入，因为子表参考的父表。

mysql> insert provinces(pname) VALUES('A');
Query OK, 1 row affected (0.03 sec)

mysql> insert provinces(pname) VALUES('b');
Query OK, 1 row affected (0.03 sec)

mysql> insert provinces(pname) VALUES('c');
Query OK, 1 row affected (0.02 sec)

mysql> SELECT * FROM provinces;
+----+-------+
| id | pname |
+----+-------+
|  1 | A     |
|  2 | b     |
|  3 | c     |
+----+-------+
3 rows in set (0.00 sec)

mysql> INSERT user1(username,pid) VALUES('Tom',1)
    -> ;
Query OK, 1 row affected (0.03 sec)

mysql> INSERT user1(username,pid) VALUES('Josn',3);
Query OK, 1 row affected (0.02 sec)

mysql> INSERT user1(username,pid) VALUES('Bob',7);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`ti`.`user1`, CONSTRAINT `user1_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `provinces` (`id`) ON DELETE CASCADE)

从父表中删除将影响子表

mysql> insert provinces(pname) VALUES('b');
Query OK, 1 row affected (0.03 sec)

mysql> insert provinces(pname) VALUES('c');
Query OK, 1 row affected (0.02 sec)

mysql> SELECT * FROM provinces;
+----+-------+
| id | pname |
+----+-------+
|  1 | A     |
|  2 | b     |
|  3 | c     |
+----+-------+
3 rows in set (0.00 sec)

mysql> INSERT user1(username,pid) VALUES('Tom',1)
    -> ;
Query OK, 1 row affected (0.03 sec)

mysql> INSERT user1(username,pid) VALUES('Josn',3);
Query OK, 1 row affected (0.02 sec)

mysql> INSERT user1(username,pid) VALUES('Bob',7);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`ti`.`user1`, CONSTRAINT `user1_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `provinces` (`id`) ON DELETE CASCADE)
mysql> INSERT user1(username,pid) VALUES('Bob',3);
Query OK, 1 row affected (0.03 sec)

mysql> SELECT *FROM USER1;
+----+----------+------+
| id | username | pid  |
+----+----------+------+
|  1 | Tom      |    1 |
|  2 | Josn     |    3 |
|  4 | Bob      |    3 |
+----+----------+------+
3 rows in set (0.00 sec)

mysql> DELETE FROM provinces where id = 3;
Query OK, 1 row affected (0.05 sec)

mysql> SELECT *FROM USER1;
+----+----------+------+
| id | username | pid  |
+----+----------+------+
|  1 | Tom      |    1 |
+----+----------+------+
1 row in set (0.00 sec)