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MySQL学习笔记约束以及修改数据表

發布時間：2025/3/21 数据库 34 豆豆

生活随笔收集整理的這篇文章主要介紹了 MySQL学习笔记约束以及修改数据表小編覺得挺不錯的,現在分享給大家,幫大家做個參考.

*** 約束保證數據的完整性和一致性

*** 約束分為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為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為參照列的

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 2	[mysqld] 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表為子表，有參照列id列provinces為父表
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:?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)   ERROR:? No?query?specified  

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:?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)   ERROR:? No?query?specified  

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，并設置子表中的外鍵列為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，稱為列級約束

*** 對多個數據列建立的約束，稱為表級約束

*** 列級約束既可以在列定義時聲明，也可以在列定義后聲明

*** 表級約束只能在列定以后聲明

4.修改數據表

添加單列

1	ALTER?TABLE?tbl_name?ADD?[COLUMN]?col_name?column_definition?[FIRST\|AFTER?col_name]

????????

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??????|????????????????| +----------+----------------------+------+-----+---------+----------------+  

????刪除列：

1	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：

1 2	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列為主鍵：

  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：

1	mysql>?ALTER?TABLE?users2?DROP?PRIMARY?KEY;

????????添加唯一約束：添加username列為唯一約束：

1	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)   ERROR:? No?query?specified  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列為外鍵列，provincse數據表中的id為參照列：

1	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：

1	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)：

1	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：

1	ALTER?TABLE?old_tbl_name?RENAME?new_tbl_name

???? ??方法二：

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

? ? ? 本文轉自謝育政 51CTO博客，原文鏈接：http://blog.51cto.com/kurolz/1876254，如需轉載請自行聯系原作者

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MySQL学习笔记 约束以及修改数据表

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MySQL学习笔记约束以及修改数据表