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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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