1、MySQL 修改字段类型或长度：
数据库中address表 city字段是varchar(30)
修改类型可以用（谨慎修改类型，可能会导致原有数据出错）
mysql> alter table address modify column city char(30);
修改长度可以用（修改长度，要保证不短与已有数据，以保证原有数据不出错）
mysql> alter table address modify column city varchar(50);2、mySQL：两表更新（用一个表更新另一个表）的SQL语句
用一个表中的字段去更新另外一个表中的字段， MySQL 中有相应的 update 语句来支持，不过这个 update 语法有些特殊。看一个例子就明白了。
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create table student
(
student_id int not null
,student_name varchar(30) not null
,city_code varchar(10) null
,city_name varchar(50) null
);
create table city
(
code varchar(10) not null
,name varchar(50) not null
);
insert into student values(1, 'john', '001', null);
insert into student values(2, 'nick', '002', null);
insert into city values('001', 'beijing');
insert into city values('002', 'shanghai');
insert into city values('003', 'shenzhen');
有两个表：student & city，现在需要取出 city.name 来更新 student.city_name。两表关联条件是 student.city_code=city.code。
update student s, city c
set s.city_name = c.name
where s.city_code = c.code;
也可以试下面的相关子查询：
update student s set city_name = (select name from city where code = s.city_code);
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1增加两个字段：
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.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)
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 www.2cto.com
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