MySQL學習筆記-約束以及修改數據表
約束：
按功能劃爲：
NOT NULL , PRIMARY KEY , UNIQUE KEY , DEFAULT , FOREIGN KEY
按數據列的數目劃爲：
表級約束，列級約束
修改數據表：
針對字段的操作：添加 / 刪除字段、修改列定義、 修改列名稱等
針對約束的操作：添加 / 刪除各種約束
針對數據表的操作：數據表更名（兩種方式）
*** 約束保證數據的完整性和一致性
*** 約束分爲表級約束和列級約束
*** 約束類型包括：
NOT NULL(非空約束)
PRIMARY KEY(主鍵約束)
UNIQUE KEY(唯一約束)
DEFAULT(默認約束)
FOREIGN KEY(外鍵約束)
1.外鍵約束的要求解析
*** 父表和子表必須使用相同的存儲引擎，而且禁止使用臨時表。
*** 數據表的存儲引擎只能爲InnoDB。
*** 外鍵列和參照列必須具有相似的數據類型。其中數字的長度或是否有符號位必須相同；而字符的長度則可以不同。
主鍵是默認自帶索引的 而外鍵列創建時一般參照的是帶有主鍵那一列 因此如果外鍵列沒有創建索引的話就會被MYSQL根據參照列的索引創建一個索引
*** 外鍵列是不可以以一個沒有索引的列作爲參照列的
1.參照列必須要創建一個索引（如果用的是主鍵默認自帶索引 所以不用創建）
2.外鍵列隨意 如果不創建索引會被創建
【1】編輯數據表的默認存儲引擎,配置文件/etc/my.cnf
1 [mysqld]2 default-storage-engine=INNODB
【2】 創建省份數據表，查看存儲引擎
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】創建用戶數據表,其中外鍵列必須和參照列必須有相似的數據類型
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：其中有外鍵列的users表爲子表，有參照列id列provinces爲父表
創建主鍵時自動創建索引，查看父表自動創建的1個索引
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)
若外鍵列沒有索引，自動創建：
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.外鍵約束的參照操作
*** cascade ：從父表刪除或更新且自動刪除或更新子表中匹配的行
*** set null ：從父表刪除或更新行，並設置子表中的外鍵列爲NULL。如果使用該選項，必須保證子表列沒有指定NOT NULL
*** restrict ：拒絕對父表的刪除或更新操作
*** no action ：標準的SQL的關鍵詞，在MySQL中與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.表級約束和列級約束
*** 對一個數據列建立的約束，稱爲列級約束
*** 對多個數據列建立的約束，稱爲表級約束
*** 列級約束既可以在列定義時聲明，也可以在列定義後聲明
*** 表級約束只能在列定以後聲明
4.修改數據表
添加單列
ALTER TABLE tbl_name(數據表名稱) ADD [COLUMN] col_name（列名） column_definition(列定義) [FIRST（插入列到最前面）|AFTER（插入列到指定列的後方） col_name(列名)（如果不寫FIRST和AFTER，則插入到最後一列）]
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列到最後一列：
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列的後面：
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 | |+----------+----------------------+------+-----+---------+----------------+
刪除列：
ALTER TABLE tbl_name(數據表名稱) DROP [COLUMN] col_name（列名）
例，刪除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 | |+----------+----------------------+------+-----+---------+----------------+
同時刪除age和password列，（也可同時增加刪除）用逗號分割：
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 / 刪除主鍵約束：
ALTER TABLE tbl_name ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...) //添加
ALTER TABLE tbl_name DROP PRIMARY KEY //刪除
添加 / 刪除唯一約束：
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 //刪除
添加 / 刪除外鍵約束：
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 //刪除
添加 / 刪除默認約束：
ALTER TABLE tbl_name ALTER [COLUMN] col_name {SET DEFAULT literal|DROP DEFAULT} //添加
mysql> ALTER TABLE tbl_name ALTER age DROP DEFAULT; //刪除
添加主鍵約束：創建users2數據表，增加id列，設置id列爲主鍵：
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 | |+----------+----------------------+------+-----+---------+-------+
刪除主鍵約束：
mysql> ALTER TABLE users2 DROP PRIMARY KEY;
添加唯一約束：添加username列爲唯一約束：
mysql> ALTER TABLE users2 ADD CONSTRAINT OK_users2_username UNIQUE KEY (username)；
刪除唯一約束：
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)
添加外鍵約束：添加pid列爲外鍵列，provincse數據表中的id爲參照列：
mysql> ALTER TABLE users2 ADD FOREIGN KEY (pid) REFERENCES provinces (id);
刪除外鍵約束：
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;
添加默認約束：添加age列，設置age列默認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 | |+----------+----------------------+------+-----+---------+-------+
刪除默認值：
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 | |+----------+----------------------+------+-----+---------+-------+
修改列定義：
ALTER TABLE tbl_name MODIFY [COLUMN] col_name column_definition [FIRST|AFTER col_name]
修改列名稱(不能隨便更改)：
ALTER TABLE tbl_name CHANGE [COLUMN] old_col_name new_col_name column_dufinition [FIRST|AFTER col_name]
修改數據表名字(不能隨便更改)：
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]...