本文主要给大家简单讲讲MySQL中该如何约束及修改数据表，相关专业术语大家可以上网查查或者找一些相关书籍补充一下，这里就不涉猎了，我们就直奔主题吧，希望MySQL中该如何约束及修改数据表这篇文章可以给大家带来一些实际帮助。
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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: PRIMARYSeq_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: PRIMARYSeq_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]...
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