MySQL之常用SQL语句

-- 将sid移动到第一行
alter table student modify sid int first;
-- 将sage移动到ssex后面
alter table student modify sage int not null after ssex;

-- 添加删除主键
alter table student add primary key(sid);
alter table student drop primary key;

-- 添加删除唯一约束1
alter table student add unique(sname);                                   
alter table student drop index sname;
alter table student drop key sname;
-- 添加唯一性约束2
alter table student add constraint uk unique key student(sname);
-- 添加联合唯一索引
alter table student add constraint double_uk unique index double_uk1(sname,ssex);

-- 添加删除自增
alter table student modify sid int auto_increment;
alter table student modify sid int;

-- 添加删除外键
alter table student add constraint fk1 foreign key(sphone) references teacher(tid);
alter table student drop foreign key fk1;

-- 添加删除非空约束
alter table student modify sage int not null;
alter table student modify sage int;

-- 修改字段名称及属性
alter table student change sssex ssex char(2) default '男';

-- 添加删除默认值
alter table student modify sssex char(2) default '男';
alter table student modify sssex char(2);                                             

-- 查看表结构
describe student;
-- 查看索引
show keys from student;
-- 查看表源码
show create table student;

-- 修改表的存储引擎
-- ALTER TABLE tb_name ENGINE=存储引擎名称
ALTER TABLE user12 ENGINE=MyISAM;

-- 修改自增长的值
-- ALTER TABLE tb_name AUTO_INCREMENT=值
ALTER TABLE user12 auto_increment=100;

-- 如果表存在就删除
drop table if exists tablename;