数据约束的分类：

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外键约束的要求解析：

//在my文件中的这句话代表着搜索引擎，如果不是的就需要技能型更改。然后重启。my文件存在于我的电脑中的mysql文件夹里。

# Thedefault storage engine that will be used when create new tables when

default-storage-engine=INNODB

root@127.0.0.1 t2>CREATE TABLEprovince(-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,-> username VARCHAR(20) NOT NULL

->);

Query OK,0 rows affected (0.13sec)

root@127.0.0.1 t2>SHOW CREATE TABLEprovince;+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table |

+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| province | CREATE TABLE`province` (

`id`smallint(5) unsigned NOT NULLAUTO_INCREMENT,

`username`varchar(20) NOT NULL,PRIMARY KEY(`id`)

) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

1 row in set (0.00sec)

root@127.0.0.1 t2>CREATE TABLEaname(-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,-> username VARCHAR(10) NOT NULL,-> pid BIGINT,-> FOREIGN KEY(pid) REFERENCESprovince(id)->);

ERROR1005 (HY000): Can't create table't2.aname'(errno: 150)

root@127.0.0.1 t2>CREATE TABLE aname(

-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,

-> username VARCHAR(10) NOT NULL,

-> pid SMALLINT UNSIGNED,

-> FOREIGN KEY(pid) REFERENCES province(id)

-> );

Query OK, 0 rows affected (0.23 sec)

root@127.0.0.1 t2>SHOW INDEXES FROMprovince\G;*************************** 1. row ***************************

Table: province

Non_unique:0Key_name:PRIMARYSeq_in_index:1Column_name: id

Collation: A

Cardinality:0Sub_part:NULLPacked:NULL

Null:

Index_type: BTREE

Comment:

Index_comment:1 row in set (0.10sec)

ERROR:

No query specified

root@127.0.0.1 t2>SHOW INDEXES FROManame\G;*************************** 1. row ***************************

Table: aname

Non_unique:0Key_name:PRIMARYSeq_in_index:1Column_name: id

Collation: A

Cardinality:0Sub_part:NULLPacked:NULL

Null:

Index_type: BTREE

Comment:

Index_comment:*************************** 2. row ***************************

Table: aname

Non_unique:1Key_name: pid

Seq_in_index:1Column_name: pid

Collation: A

Cardinality:0Sub_part:NULLPacked:NULL

Null: YES

Index_type: BTREE

Comment:

Index_comment:2 rows in set (0.00sec)

ERROR:

No query specified

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外键约束的参照操作：

root@127.0.0.1 t2>CREATE TABLEaname1(-> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,-> username VARCHAR(10) NOT NULL,-> pid SMALLINTUNSIGNED,-> FOREIGN KEY(pid) REFERENCES province(id) ON DELETE CASCADE

->);

Query OK,0 rows affected (0.23sec)

root@127.0.0.1 t2>SHOW CREATE TABLEaname1;+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table |

+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

| aname1 | CREATE TABLE`aname1` (

`id`smallint(5) unsigned NOT NULLAUTO_INCREMENT,

`username`varchar(10) NOT NULL,

`pid`smallint(5) unsigned DEFAULT NULL,PRIMARY KEY(`id`),KEY`pid` (`pid`),CONSTRAINT `aname1_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `province` (`id`) ON DELETE CASCADE) ENGINE=InnoDB DEFAULT CHARSET=utf8 |

+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

1 row in set (0.00 sec)

root@127.0.0.1 t2>INSERT province(username) VALUES('A');

Query OK,1 row affected (0.16sec)

root@127.0.0.1 t2>INSERT province(username) VALUES('B');

Query OK,1 row affected (0.14sec)

root@127.0.0.1 t2>INSERT province(username) VALUES('C');

Query OK,1 row affected (0.13sec)

root@127.0.0.1 t2>SELECT * FROMprocvince;

ERROR1146 (42S02): Table 't2.procvince' doesn't exist

root@127.0.0.1 t2>SELECT * FROM province;

+----+----------+

| id | username |

+----+----------+

| 1 | A |

| 2 | B |

| 3 | C |

+----+----------+

3 rows in set (0.00 sec)

root@127.0.0.1 t2>INSERT aname1(username,pid) VALUES('D',3);

Query OK, 1 row affected (0.16 sec)

root@127.0.0.1 t2>INSERT aname1(username,pid) VALUES('E',1);

Query OK, 1 row affected (0.07 sec)

root@127.0.0.1 t2>INSERT aname1(username,pid) VALUES('E',t);

ERROR 1054 (42S22): Unknown column't'in'field list'root@127.0.0.1 t2>INSERT aname1(username,pid) VALUES('F',7);

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`t2`.`aname1`, CONSTRAINT `aname1_ibfk_1` FOREIGN KEY (`pid`) REFERENCES `province` (`id`) ON DELETE CASCADE)

root@127.0.0.1 t2>INSERT aname1(username,pid) VALUES('F',2);

Query OK, 1 row affected (0.05 sec)

root@127.0.0.1 t2>SELECT * FROM province;

+----+----------+

| id | username |

+----+----------+

| 1 | A |

| 2 | B |

| 3 | C |

+----+----------+

3 rows in set (0.00 sec)

root@127.0.0.1 t2>SELECT * FROM aname1;

+----+----------+------+

| id | username | pid |

+----+----------+------+

| 1 | D | 3 |

| 2 | E | 1 |

| 4 | F | 2 |

+----+----------+------+

3 rows in set (0.00 sec)

root@127.0.0.1 t2>DELETE FROM province where id = 3;

Query OK, 1 row affected (0.20 sec)

root@127.0.0.1 t2>SELECT * FROM province;

+----+----------+

| id | username |

+----+----------+

| 1 | A |

| 2 | B |

+----+----------+

2 rows in set (0.00 sec)

root@127.0.0.1 t2>SELECT * FROM aname1;

+----+----------+------+

| id | username | pid |

+----+----------+------+

| 2 | E | 1 |

| 4 | F | 2 |

+----+----------+------+

2 rows in set (0.00 sec)

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表级约束以及列级约束：

在实际开发的时候我们使用列级约束的较多，表级约束较少。default和not null只存在列级约束，其他的都存在表级约束。

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修改数据表增加和删除列：

添加(删除)列 alter table +表明drop+列名

添加(删除)多列 alter table +表明 【drop+列名，drop+列名】；(在添加单列的时候所有的列不需要指明小括号，在指明多列的时候，我们不能指定位子)

添加单列alert table tbl_name add +列名字+column_definition [firstafter col_name]

root@127.0.0.1 t2>ALTER TABLE aname1 ADD age TINYINT UNSIGNED NOT NULL DEFAULT 10;

Query OK,2 rows affected (0.34sec)

Records:2 Duplicates: 0 Warnings: 0root@127.0.0.1 t2>SHOW COLUMNS FROManame1;+----------+----------------------+------+-----+---------+----------------+

| 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 | |

+----------+----------------------+------+-----+---------+----------------+

4 rows in set (0.01 sec)

root@127.0.0.1 t2>ALTER TABLE aname1 ADD ab TINYINT UNSIGNED NOT NULLAFTER username;

Query OK,2 rows affected (0.30sec)

Records:2 Duplicates: 0 Warnings: 0root@127.0.0.1 t2>SHOW COLUMNS FROManame1;+----------+----------------------+------+-----+---------+----------------+

| Field | Type | Null | Key | Default | Extra |

+----------+----------------------+------+-----+---------+----------------+

| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |

| username | varchar(10) | NO | | NULL | |

| ab | tinyint(3) unsigned | NO | | NULL | |

| pid | smallint(5) unsigned | YES | MUL | NULL | |

| age | tinyint(3) unsigned | NO | | 10 | |

+----------+----------------------+------+-----+---------+----------------+

5 rows in set (0.02 sec)

在我们进行操作的时候我们可以同时的进行删除和增加的操作，我们只需要在两个操作之间用‘，’进行分隔就好了。

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修改数据表添加约束：

添加外键约束：ALTER TABLE name ADD FOREIGN KEY(pid) REFERENCES provinces (id);

添加默认约束：ALTER TABLE name ALTER age SET DEFAULT 15;

删除默认约束：ALTER TABLE name ALTER age DROP DEFAULT;

数据表的修改操作：无非就是添加列，删除列，添加约束，删除约束。用的是ALTER，而INSERT是对数据表添加插入记录用的

1、添加主键约束：

ALTER TABLE table_name ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_column_name,...);

2、添加单个唯一约束：

ALTER TABLE table_name ADD [CONSTRAINT [symbol]] UNIQUE [INDEXKEY] [index_name] [index_type] (index_column_name,...);

3、添加多个唯一约束：

ALTER TABLE table_name ADD [CONSTRAINT [symbol]] UNIQUE [INDEXKEY] [index_name] [index_type] (index_column_name,...),ADD [CONSTRAINT [symbol]] UNIQUE [INDEXKEY] [index_name] [index_type] (index_column_name,...);

Ps1：CONSTRAINT 可加可不加，加该关键字后还可以选择添加主键别名；

Ps2：唯一约束可以有多个，但主键约束有且只能有一个。

root@127.0.0.1 t2>ALTER TABLE user2 ADD CONSTRAINT id PRIMARY KEY(id);

Query OK,0 rows affected (0.34sec)

Records:0 Duplicates: 0 Warnings: 0root@127.0.0.1 t2>SHOW COLUMNS fromuser2;+----------+----------------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+----------------------+------+-----+---------+-------+

| id | smallint(5) unsigned | NO | PRI | 0 | |

| username | varchar(20) | NO | | NULL | |

| pid | smallint(5) unsigned | NO | | NULL | |

+----------+----------------------+------+-----+---------+-------+

3 rows in set (0.00 sec)

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修改数据表—删除约束：

1、删除主键约束：

ALTER TABLE table_name DROP PRIMARY KEY;

2、删除唯一约束：

ALTER TABLE table_name DROP {INDEX} key_name;

3、删除外键约束：

ALTER TABLE table_name DROP FOREIGN KEY fk_symbol;

Ps1：唯一约束的 index_name 可通过 SHOW INDEX FROM table_name\G; 查询。

Ps2：外键约束的名字 fk_symbol 可通过 SHOW CREATE TABLE table_name; 查询。

Ps3：INDEX：index是索引标识，和create table name 中的table是相同的标识。

Ps4：当字段id smallint unsigned auto_increment primary key时，不可以删除主键约束；必须先修改为 id smallint unsigned，再删除主键约束。

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修改列定义以及更名数据表：

修改列定义和更名数据表1、修改列定义(列类型/列位置)ALTER TABLE tbl_name MODIFY [COLUMN] col_name column_definition [FIRST AFTER col_name];2、修改列名称ALTER TABLE tbl_name CHANGE [COLUMN] col_name new_col_name column_definition [FIRSTAFTER col_name];3、数据表更名

方法1：ALTER TABLE tbl_name RENAME [TO/AS]new_tbl_name

方法2：

RENAMETABLE tbl_name TO new_tbl_name [, tbl_name2 TO new_tbl_name2]...(这种方法可以给多个数据表更名)

Ps1：修改数据类型(特别是大类型转到小类型),需注意数据丢失的问题。

Ps2：尽量不要修改数据表名和列名，以免影响后台等问题。

root@127.0.0.1 t2>SHOW COLUMNS fromtb3;+----------+----------------------+------+-----+---------+----------------+

| Field | Type | Null | Key | Default | Extra |

+----------+----------------------+------+-----+---------+----------------+

| id | smallint(5) unsigned | NO | PRI | NULL | auto_increment |

| username | varchar(30) | NO | | NULL | |

+----------+----------------------+------+-----+---------+----------------+

2 rows in set (0.02sec)

root@127.0.0.1 t2>ALTER TABLE tb3 MODIFY id SMALLINT UNSIGNED NOT NULL;

Query OK,0 rows affected (0.34sec)

Records:0 Duplicates: 0 Warnings: 0root@127.0.0.1 t2>SHOW COLUMNS fromtb3;+----------+----------------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+----------+----------------------+------+-----+---------+-------+

| id | smallint(5) unsigned | NO | PRI | NULL | |

| username | varchar(30) | NO | | NULL | |

+----------+----------------------+------+-----+---------+-------+

2 rows in set (0.01 sec)

我们在修改定义的时候我们不需要带上主键的名称。