mysql（7）分组查询和分页查询

同时查询多条数据：

select * from students where id =1 or id=2 or id=3;

select * from students where id in (1,2,3);

select * from students where id not in(2,3);

同时查询数据+ 排序：
select * from students where id in (2,3) order by python_grade ASC;

分组查询： group by :

create table goods(  id bigint(20) not null auto_increment primary key , goodName varchar(40) , goodCategory varchar(20) );

insert into goods (goodName, goodCategory ) values('apple','phone') ,('oppo' ,'phone' ),('mac' ,'computer' ),('lenovo' ,'computer'),('vivo','phone');

查询goods商品的种类：

select goodCategory from goods group by goodCategory;

查询goods商品是否存在类型
gruop by +having

select goodCategory from goods group by goodCategory having goodCategory='phone';

分页：（limit 起始下标,每页显示数据量）
goods 表中有7条数据，每页显示3条，一共可以显示3页；

获取第一页数据： select * from goods limit 0,3;

获取第二页数据： select * from goods limit 3,3;

获取第三页数据：select * from goos limit 6,3;

select * from goods limit (pageNumber-1)*pageSize,pageSize;