Mysql命令增加、修改、删除表字段

一、方式一

1、alter add 命令用来增加表的字段

alter add命令格式：alter table 表名 add字段 类型 其他；

alter table car_evidence add `unit_name` varchar(64) comment '单位'
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2、alter drop 命令删除表的字段

alter drop 命令格式：alter table 表名 drop column 字段；

alter table car_evidence drop column `unit_name`; 
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3、alter modify 命令修改表的字段

alter modify 命令格式：alter table 表名 modify 字段 其他；

alter table t_plan MODIFY box_size VARCHAR(32) comment '盒内的数量';
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二、方式二

1.增加一个字段

alter table user add COLUMN new1 VARCHAR(20) DEFAULT NULL; //增加一个字段，默认为空
alter table user add COLUMN new2 VARCHAR(20) NOT NULL; //增加一个字段，默认不能为空

2.删除一个字段

alter table user DROP COLUMN new2; 　//删除一个字段

3.修改一个字段

alter table user MODIFY new1 VARCHAR(10); 　//修改一个字段的类型
alter table user CHANGE new1 new4 int;　//修改一个字段的名称，此时一定要重新指定该字段的类型

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

1.增加两个字段
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.删除两个字段
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.查询看一下
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.新增字段并插入
mysql> insert into id_name values(2,'qust',14,'山东');
Query OK, 1 row affected (0.00 sec)

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