MySQL中的表中增長刪除字段

mysql> create table id_name(id int,name varchar(20));
Query OK, 0 rows affected (0.13 sec)


mysql> alter table id_name add age int,add address varchar(11);
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc id_name；

+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id      | int(11)     | YES  |     | NULL    |       |
| name    | varchar(20) | YES  |     | NULL    |       |
| age     | int(11)     | YES  |     | NULL    |       |
| address | varchar(11) | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+
4 rows in set (0.00 sec)

2.刪除兩個字段
mysql> alter table id_name drop column age,drop column address;
Query OK, 0 rows affected (0.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc id_name;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

3.插入
mysql> insert into id_name values (1,'qustdjx');
Query OK, 1 row affected (0.00 sec)
4.查詢看一下
mysql> alter table id_name add age int,add address varchar(11);
Query OK, 1 row affected (0.07 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> select * from id_name;
+------+---------+------+---------+
| id   | name    | age  | address |
+------+---------+------+---------+
|    1 | qustdjx | NULL | NULL    |
+------+---------+------+---------+
1 row in set (0.00 sec)
5.新增字段並插入
mysql> insert into id_name values(2,'qust',14,'山東');
Query OK, 1 row affected (0.00 sec)

mysql> select * from id_name;
+------+---------+------+---------+
| id   | name    | age  | address |
+------+---------+------+---------+
|    1 | qustdjx | NULL | NULL    |
|    2 | qust    |   14 | 山東    |
+------+---------+------+---------+
2 rows in set (0.00 sec)

 

1.增長一個字段
alter table user add COLUMN new1 VARCHAR(20) DEFAULT NULL; //增長一個字段，默認爲空
alter table user add COLUMN new2 VARCHAR(20) NOT NULL;  //增長一個字段，默認不能爲空  www.2cto.com  
 
2.刪除一個字段
alter table user DROP COLUMN new2; 　 //刪除一個字段
 
3.修改一個字段
alter table user MODIFY new1 VARCHAR(10); 　//修改一個字段的類型
 
alter table user CHANGE new1 new4 int;　 //修改一個字段的名稱，此時必定要從新指定該字段的類型