CRUD操作：
create 创建（添加）
read 读取
update 修改
delete 删除
4、读取数据
(1)简单读取,查询所有列(*) 所有行（没有加条件）
select * from Info
(2)读取特定列
select code,name from Info
(3)条件查询
select * from Info where code='p003'
(4)多条件查询
select * from Info where code='p003' or nation='n002' #或的关系
select * from Info where sex=0 and nation='n002' #与的关系
(5)关键字查询（模糊查询）
查所有包含奥迪的汽车
select * from car where name like '%奥迪%'; #百分号%代表任意多个字符
查以'皇冠'开头的所有汽车
select * from car where name like '皇冠%';
查询汽车名称中第二个字符是'马'的
select * from car where name like '_马%'; #下划线_代表任意一个字符
(6)排序查询
select * from car order by powers #默认升序排列
select * from car order by powers desc #升序asc 降序 desc
先按brand升序排，再按照price降序排
select * from car order by brand,price desc
(7）范围查询
select * from car where price > 50 and price<60
select* from car where between 40 and 60
(8)离散查询
select * from where price=30 and price=40 and price=50 and price=60
select * from where price in (30,40,50,60)
select * from where price not in (30,40,50,60)
(9)统计查询（聚合函数）
select count（*） from car car表里所有的记录
*可以用主键列代替，更快
求和：select sum(price) from car
平均：select avg(price) from car
最大值：select max(price) from car
最小值：select min(price) from car
(10)分页查询
select * from car limit 0,10 跳过0条数据，取10条
规定一个每页显示的条数：m
当前页数 ： n
select * from car limit (n-1)*m,m
(11)去重查询 ***适合查一列***
select distinct brand from car ***distinct去重***
（12）分组查询
查询汽车表中，每个系列下汽车的数量
select brand,count(*) from car group by brand
分组完成后，只能取分完组的列
select brand from car group by brand having avg(price) >40