mysql之约束以及修改数据表

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。
# The default storage engine that will be used when create new tables when
default-storage-engine=INNODB

root@127.0.0.1 t2>CREATE TABLE province(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(20) NOT NULL
    -> );
Query OK, 0 rows affected (0.13 sec)

root@127.0.0.1 t2>SHOW CREATE TABLE province;
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| province | CREATE TABLE `province` (
  `id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
  `username` varchar(20) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+----------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

root@127.0.0.1 t2>CREATE TABLE aname(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(10) NOT NULL,
    -> pid BIGINT ,
    -> FOREIGN KEY(pid) REFERENCES province(id)
    -> );
ERROR 1005 (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 FROM province\G;
*************************** 1. row ***************************
        Table: province
   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:
Index_comment:
1 row in set (0.10 sec)

ERROR:
No query specified

root@127.0.0.1 t2>SHOW INDEXES FROM aname\G;
*************************** 1. row ***************************
        Table: aname
   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:
Index_comment:
*************************** 2. row ***************************
        Table: aname
   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:
Index_comment:
2 rows in set (0.00 sec)

ERROR:
No query specified

——————————————————————————————————————————————————————————————————

外键约束的参照操作：

root@127.0.0.1 t2>CREATE TABLE aname1(
    -> id SMALLINT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(10) NOT NULL,
    -> pid SMALLINT UNSIGNED,
    -> FOREIGN KEY(pid) REFERENCES province(id) ON DELETE CASCADE
    -> );
Query OK, 0 rows affected (0.23 sec)

root@127.0.0.1 t2>SHOW CREATE TABLE aname1;
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table  | Create Table                                                                                                                                                                                                                                                                                                                               |
+--------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| aname1 | CREATE TABLE `aname1` (
  `id` smallint(5) unsigned NOT NULL AUTO_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 (0.16 sec)

root@127.0.0.1 t2>INSERT province(username) VALUES('B');
Query OK, 1 row affected (0.14 sec)

root@127.0.0.1 t2>INSERT province(username) VALUES('C');
Query OK, 1 row affected (0.13 sec)

root@127.0.0.1 t2>SELECT * FROM procvince;
ERROR 1146 (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)

——————————————————————————————————————————————————————————————————

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 (0.34 sec)
Records: 2  Duplicates: 0  Warnings: 0

root@127.0.0.1 t2>SHOW COLUMNS FROM aname1;
+----------+----------------------+------+-----+---------+----------------+
| 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 NULL AFTER username;
Query OK, 2 rows affected (0.30 sec)
Records: 2  Duplicates: 0  Warnings: 0

root@127.0.0.1 t2>SHOW COLUMNS FROM aname1;
+----------+----------------------+------+-----+---------+----------------+
| 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 (0.34 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@127.0.0.1 t2>SHOW COLUMNS from user2;
+----------+----------------------+------+-----+---------+-------+
| 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：
RENAME TABLE 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 from tb3;
+----------+----------------------+------+-----+---------+----------------+
| Field    | Type                 | Null | Key | Default | Extra          |
+----------+----------------------+------+-----+---------+----------------+
| id       | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(30)          | NO   |     | NULL    |                |
+----------+----------------------+------+-----+---------+----------------+
2 rows in set (0.02 sec)

root@127.0.0.1 t2>ALTER TABLE tb3 MODIFY id SMALLINT UNSIGNED NOT NULL;
Query OK, 0 rows affected (0.34 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@127.0.0.1 t2>SHOW COLUMNS from tb3;
+----------+----------------------+------+-----+---------+-------+
| 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。