Mysql

1.增加/删除一列

alter table tablename drop column columnname;

alter table tabelname add columnname varchar(8) NULL; 

2.mysql如何设置密码 
.用root 进入mysql后
mysql>set password =password('你的密码');
mysql>flush privileges;

.使用GRANT语句
mysql>grant all on *.* to 'root'@'localhost' IDENTIFIED BY '你的密码'with grant option ;
mysql>flush privileges;

.进入mysql库修改user表
mysql>use mysql;
mysql>update user set password=password('你的密码') where user='root';
mysql>flush privileges;

.设置密码为空

mysql>use mysql;
mysql>update user set password=password('') where user='root';
mysql>flush privileges;