MySQL之修改表中的列

MySQL之修改表中的列

修改表中列的语法：

一张表，创建完毕，有了N列。之后还可以增加或删除或修改列

alter table 表名 add 列名称 列类型 列参数; [这样加的列在表的最后]

例:alter table m1 add username char(20) not null default '';

alter table 表名 add 列名称 列类型 列参数 after 某列; [新列加在某列后]

例:alter table m1 add username char(20) not null default '';

如果想增加一列，并位于表的最前面，用first

alter table 表名 add 列名称 列类型 列参数 first;

例:alter table m1 add pid int not null first;

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删除表中列的语法：

alter table 表名 drop 列名;

例:alter table m1 drop pid;

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修改表中列类型的语法：

alter table 表名 modify 列名 新类型 新参数;

例:alter table m1 modify gender char(4) not null default '';

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修改表中列名及类型的语法：

alter table 表名 change 旧列名 新列名 新类型 新参数;

例:alter table m1 change id uid int unsigned;

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下面是在mysql中的操作：

mysql> create table m1(

-> id int unsigned auto_increment primary key

-> );

Query OK, 0 rows affected (0.62 sec)

mysql> desc m1;

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

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

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

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

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

1 row in set (0.14 sec)

mysql> alter table m1 add username char(20) not null default '';

Query OK, 0 rows affected (0.49 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

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

2 rows in set (0.01 sec)

mysql> alter table m1 add birth date not null default '0000-00-00';

Query OK, 0 rows affected (0.46 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

3 rows in set (0.01 sec)

mysql> alter table m1 add gender char(1) not null default '' after username;

Query OK, 0 rows affected (0.36 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

| gender | char(1) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

4 rows in set (0.01 sec)

mysql> #如果想增加一列，并位于表的最前面，用first

mysql> alter table m1 add pid int not null default '' first;

ERROR 1067 (42000): Invalid default value for 'pid'

mysql> alter table m1 add pid int not null first;

Query OK, 0 rows affected (0.40 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| pid | int(11) | NO | | NULL | |

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

| gender | char(1) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

5 rows in set (0.01 sec)

mysql> #删除列

mysql> desc m1;

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

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

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

| pid | int(11) | NO | | NULL | |

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

| gender | char(1) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

5 rows in set (0.01 sec)

mysql> alter table m1 drop pid;

Query OK, 0 rows affected (0.37 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

| gender | char(1) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

4 rows in set (0.01 sec)

mysql> alter table m1 modify gender char(4) not null default '';

Query OK, 0 rows affected (0.47 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| id | int(10) unsigned | NO | PRI | NULL | auto_increment |

| username | char(20) | NO | | | |

| gender | char(4) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

4 rows in set (0.05 sec)

mysql> alter table m1 change id uid int unsigned;

Query OK, 0 rows affected (0.44 sec)

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc m1;

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

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

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

| uid | int(10) unsigned | NO | PRI | 0 | |

| username | char(20) | NO | | | |

| gender | char(4) | NO | | | |

| birth | date | NO | | 0000-00-00 | |

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

4 rows in set (0.01 sec)

mysql> exit;

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