MySQL学习笔记-约束以及修改数据表

MySQL学习笔记-约束以及修改数据表

约束：
按功能划为：
NOT NULL , PRIMARY KEY , UNIQUE KEY , DEFAULT , FOREIGN KEY
按数据列的数目划为：
表级约束，列级约束
修改数据表：
针对字段的操作：添加 / 删除字段、修改列定义、 修改列名称等
针对约束的操作：添加 / 删除各种约束
针对数据表的操作：数据表更名（两种方式）

*** 约束保证数据的完整性和一致性

*** 约束分为表级约束和列级约束

*** 约束类型包括：

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)

1.外键约束的要求解析

*** 父表和子表必须使用相同的存储引擎，而且禁止使用临时表。

*** 数据表的存储引擎只能为InnoDB。

*** 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位必须相同；而字符的长度则可以不同。

主键是默认自带索引的 而外键列创建时一般参照的是带有主键那一列 因此如果外键列没有创建索引的话就会被MYSQL根据参照列的索引创建一个索引

*** 外键列是不可以以一个没有索引的列作为参照列的

1.参照列必须要创建一个索引（如果用的是主键默认自带索引 所以不用创建）

2.外键列随意 如果不创建索引会被创建

【1】编辑数据表的默认存储引擎,配置文件/etc/my.cnf

1 [mysqld]
2 default-storage-engine=INNODB

【2】 创建省份数据表，查看存储引擎

mysql> USE test;
mysql> CREATE TABLE provinces(
-> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
-> pname VARCHAR(20) NOT NULL
-> );
mysql> SHOW CREATE TABLE provinces;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                        |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb0   | CREATE TABLE `provinces` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`pname` varchar(20) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+

【3】创建用户数据表,其中外键列必须和参照列必须有相似的数据类型

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)
-> );

注意：其中有外键列的users表为子表，有参照列id列provinces为父表

创建主键时自动创建索引，查看父表自动创建的1个索引

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:
1 row in set (0.00 sec)

若外键列没有索引，自动创建：

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:
*************************** 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:
2 rows in set (0.00 sec)

2.外键约束的参照操作

*** 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 ：拒绝对父表的删除或更新操作

*** no action ：标准的SQL的关键词，在MySQL中与restrict相同

CASCADE例：

mysql> CREATE TABLE provinces(
-> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
-> pname VARCHAR(20) NOT NULL
-> );

mysql> CREATE TABLE users1(
-> id SMALLINT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
-> username VARCHAR(10) NOT NULL,
-> pid SMALLINT UNSIGNED,
-> FOREIGN KEY (pid) REFERENCES provinces (id) ON DELETE CASCADE
-> );

mysql> INSERT provinces(pname) VALUES('Tom');
mysql> INSERT provinces(pname) VALUES('John');
mysql> INSERT provinces(pname) VALUES('Driver');

mysql> INSERT users1(username,pid) VALUES('Huang',2);
mysql> INSERT users1(username,pid) VALUES('Li',3);
mysql> INSERT users1(username,pid) VALUES('Pan',3);
mysql> INSERT users1(username,pid) VALUES('He',1);
mysql> INSERT users1(username,pid) VALUES('Long',2);
mysql> SELECT * FROM users1;
+----+----------+------+
| id | username | pid  |
+----+----------+------+
|  1 | Huang    |    2 |
|  2 | Li       |    3 |
|  5 | Pan      |    3 |
|  6 | He       |    1 |
|  7 | Long     |    2 |
+----+----------+------+
mysql> SELECT * FROM provinces;
+----+--------+
| id | pname  |
+----+--------+
|  1 | Tom    |
|  2 | John   |
|  3 | Driver |
+----+--------+

mysql> DELETE FROM provinces WHERE id=3;
mysql> SELECT * FROM provinces;
+----+-------+
| id | pname |
+----+-------+
|  1 | Tom   |
|  2 | John  |
+----+-------+
mysql> SELECT * FROM users1;
+----+----------+------+
| id | username | pid  |
+----+----------+------+
|  1 | Huang    |    2 |
|  6 | He       |    1 |
|  7 | Long     |    2 |
+----+----------+------+

3.表级约束和列级约束

*** 对一个数据列建立的约束，称为列级约束

*** 对多个数据列建立的约束，称为表级约束

*** 列级约束既可以在列定义时声明，也可以在列定义后声明

*** 表级约束只能在列定以后声明

4.修改数据表

添加单列

ALTER TABLE tbl_name(数据表名称) ADD [COLUMN] col_name（列名） column_definition(列定义) [FIRST（插入列到最前面）|AFTER（插入列到指定列的后方） col_name(列名)（如果不写FIRST和AFTER，则插入到最后一列）]

例：

mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(10)          | NO   |     | NULL    |                |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+

添加age列到最后一列：

mysql> ALTER TABLE users1 ADD age TINYINT UNSIGNED NOT NULL DEFAULT 10;
mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(10)          | NO   |     | NULL    |                |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |                |
| age      | tinyint(3) unsigned  | NO   |     | 10      |                |
+----------+----------------------+------+-----+---------+----------------+

添加password列到username列的后面：

mysql> ALTER TABLE users1 ADD password VARCHAR(32) NOT NULL AFTER username;
mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(10)          | NO   |     | NULL    |                |
| password | varchar(32)          | NO   |     | NULL    |                |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |                |
| age      | tinyint(3) unsigned  | NO   |     | 10      |                |
+----------+----------------------+------+-----+---------+----------------+

添加truename列到第一列：

mysql> ALTER TABLE users1 ADD truename VARCHAR(32) NOT NULL FIRST;
mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| truename | varchar(32)          | NO   |     | NULL    |                |
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(10)          | NO   |     | NULL    |                |
| password | varchar(32)          | NO   |     | NULL    |                |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |                |
| age      | tinyint(3) unsigned  | NO   |     | 10      |                |
+----------+----------------------+------+-----+---------+----------------+

删除列：

ALTER TABLE tbl_name(数据表名称) DROP [COLUMN] col_name（列名）

例，删除truename列：

mysql> ALTER TABLE users1 DROP truename;
mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(10)          | NO   |     | NULL    |                |
| password | varchar(32)          | NO   |     | NULL    |                |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |                |
| age      | tinyint(3) unsigned  | NO   |     | 10      |                |
+----------+----------------------+------+-----+---------+----------------+

同时删除age和password列，（也可同时增加删除）用逗号分割：

mysql> ALTER TABLE users1 DROP age,DROP password;
mysql> SHOW COLUMNS FROM users1;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(10)          | NO   |     | NULL    |                |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+

添加 / 删除主键约束：

ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...)       //添加

ALTER TABLE tbl_name DROP PRIMARY KEY     //删除

添加 / 删除唯一约束：

ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...)    //添加

ALTER TABLE tbl_name DROP {INDEX|KEY} index_name   //删除

添加 / 删除外键约束：

ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,...) reference_definition    //添加

ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol   //删除

添加 / 删除默认约束：

ALTER TABLE tbl_name ALTER [COLUMN] col_name {SET DEFAULT literal|DROP DEFAULT}    //添加
mysql> ALTER TABLE tbl_name ALTER age DROP DEFAULT;    //删除

添加主键约束：创建users2数据表，增加id列，设置id列为主键：

mysql> CREATE TABLE users2(
-> username VARCHAR(20) NOT NULL,
-> pid SMALLINT UNSIGNED
-> );
mysql> ALTER TABLE users2 ADD id SMALLINT UNSIGNED;
mysql> ALTER TABLE users2 ADD CONSTRAINT pk_users2_id PRIMARY KEY (id);
mysql> SHOW COLUMNS FROM users2;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| username | varchar(20)          | NO   |     | NULL    |       |
| pid      | smallint(5) unsigned | YES  |     | NULL    |       |
| id       | smallint(5) unsigned | NO   | PRI | 0       |       |
+----------+----------------------+------+-----+---------+-------+

删除主键约束：

mysql> ALTER TABLE users2 DROP PRIMARY KEY;

添加唯一约束：添加username列为唯一约束：

mysql> ALTER TABLE users2 ADD CONSTRAINT OK_users2_username UNIQUE KEY (username)；

删除唯一约束：

mysql> SHOW INDEXES FROM users2\G;
*************************** 1. row ***************************
Table: users2
Non_unique: 0
Key_name: OK_users2_username
Seq_in_index: 1
Column_name: username
Collation: A
Cardinality: 0
Sub_part: NULL
Packed: NULL
Null:
Index_type: BTREE
Comment:
*************************** 2. row ***************************
Table: users2
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:
2 rows in set (0.00 sec)

mysql> ALTER TABLE users2 DROP INDEX OK_users2_username;
mysql> SHOW INDEXES FROM users2\G;
*************************** 1. row ***************************
Table: users2
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:
1 row in set (0.00 sec)

添加外键约束：添加pid列为外键列，provincse数据表中的id为参照列：

mysql> ALTER TABLE users2 ADD FOREIGN KEY (pid) REFERENCES provinces (id);

删除外键约束：

mysql> SHOW CREATE TABLE users2;
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                                                                                                                  |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| users2 | CREATE TABLE `users2` (
`username` varchar(20) NOT NULL,
`pid` smallint(5) unsigned DEFAULT NULL,
`id` smallint(5) unsigned NOT NULL DEFAULT '0',
`age` tinyint(4) NOT NULL,
KEY `pid` (`pid`),
CONSTRAINT `users2_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `provinces` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+--------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

mysql> ALTER TABLE users2 DROP FOREIGN KEY users2_ibfk_1;

添加默认约束：添加age列，设置age列默认15

mysql> ALTER TABLE users2 ADD age TINYINT NOT NULL;
mysql> SHOW COLUMNS FROM users2;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| username | varchar(20)          | NO   | UNI | NULL    |       |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |       |
| id       | smallint(5) unsigned | NO   | PRI | 0       |       |
| age      | tinyint(4)           | NO   |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+
mysql> ALTER TABLE users2 ALTER age SET DEFAULT 15;
mysql> SHOW COLUMNS FROM users2;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| username | varchar(20)          | NO   | UNI | NULL    |       |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |       |
| id       | smallint(5) unsigned | NO   | PRI | 0       |       |
| age      | tinyint(4)           | NO   |     | 15      |       |
+----------+----------------------+------+-----+---------+-------+

删除默认值：

mysql> ALTER TABLE users2 ALTER age DROP DEFAULT;
mysql> SHOW COLUMNS FROM users2;
+----------+----------------------+------+-----+---------+-------+
| Field    | Type                 | Null | Key | Default | Extra |
+----------+----------------------+------+-----+---------+-------+
| username | varchar(20)          | NO   | UNI | NULL    |       |
| pid      | smallint(5) unsigned | YES  | MUL | NULL    |       |
| id       | smallint(5) unsigned | NO   | PRI | 0       |       |
| age      | tinyint(4)           | NO   |     | NULL    |       |
+----------+----------------------+------+-----+---------+-------+

修改列定义：

ALTER TABLE tbl_name MODIFY [COLUMN] col_name column_definition [FIRST|AFTER col_name]

修改列名称(不能随便更改)：

ALTER TABLE tbl_name CHANGE [COLUMN] old_col_name new_col_name column_dufinition [FIRST|AFTER col_name]

修改数据表名字(不能随便更改)：

方法一：

ALTER TABLE old_tbl_name RENAME new_tbl_name

方法二：

RENAME TABLE old_tbl_name TO new_tbl_name [,tbl_name2 TO new_tbl_name2]...