MySQL之修改表中的列

MySQL之修改表中的列

MySQL之修改表中的列

修改表中列的語法：

一張表，創建完畢，有了N列。之後還可以增加或刪除或修改列

alter table 表名 add 列名稱 列類型 列參數; [這樣加的列在表的最後]
例:alter table m1 add username char(20) not null default '';

alter table 表名 add 列名稱 列類型 列參數 after 某列; [新列加在某列後]
例:alter table m1 add username char(20) not null default '';

如果想增加一列，並位於表的最前面，用first
alter table 表名 add 列名稱 列類型 列參數 first;
例:alter table m1 add pid int not null first;
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刪除表中列的語法：

alter table 表名 drop 列名;
例:alter table m1 drop pid;
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修改表中列類型的語法：

alter table 表名 modify 列名 新類型 新參數;
例:alter table m1 modify gender char(4) not null default '';
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修改表中列名及類型的語法：

alter table 表名 change 舊列名 新列名 新類型 新參數;
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，並位於表的最前面，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> #刪除列
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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歡迎探討與學習。。。。。。