数据库_CRUD操作之读取数据

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