一，数据库基本操作，连表查询

show databases;
-- `显示所有的库`
use koa2weibo;
-- `启用当前的库`
 
select * from blog order by id desc;  //通过id 倒序查询
select count(*) from blog; //查询总数
select count(*) as `count` from blog; //查询总数用一个别名
select count(id) as `count` from blog;//查询总数不需要查所有的 查id就行了

select * from blog order by id desc limit 2 offset 2;  //查表进行分页每页limit2条 

select * from users; //查询users表 所有数据 一般不用 而用下面条件查询
select username,`password` from users where username='aaa' and `password`='123'; //条件查询
-- select username,password from users  //条件查询

insert into users (username,`password`,nickname) values('aaa','123','二个eee猴子'); //新增
insert into blog (title,content,userid) values('标题4','内容4',2); //新增
update blog set content='内容sssss' where id ='1'; //更新
delete from blog where id = '3' //删除

-- '连表查询'
select * from blog inner join users on users.id = blog.userid;

select blog.*,users.username,users.nickname from blog inner join users on users.id = blog.userid;

二,ORM知识点

直接在node里面写sql语句查询比较底层，也不太方便。用ORM（对象关系映射）工具简化操作。

我们用sequelize

npm i mysql2 sequelize -D