1202数据库查询

Posted 2020-08-21 王小明爱红领巾

1. create database test
2. drop database test
3. create table info
(
code int primary key,
name varchar(20) not null
)
auto_increment 自增长列
foreign key(列名) references 主表名(列名) 外键关系

4. drop table info

CRUD:
1.insert into 表名(列名) values(值)

2.delete from 表名 where 条件

3.update 表名 set 列名=值 where 条件

简单查询

1.最简单查询（查所有数据）
select * from 表名； 注：* 代表所有列
select * from info

2.查询指定列
select code,name from info

3.修改结果集的列名
select code as ‘代号‘,name as ‘姓名‘ from info

4.条件查询
select * from info where code=‘p003‘

5.多条件查询
查询info表中code为p003或者nation为n001的所有数据
select * from info where code=‘p003‘ or nation=‘n001‘
查询info表中code为p004并且nation为n001的数据
select * from info where code=‘p004‘ and nation=‘n001‘

6.范围查询
select * from car where price>=40 and price<=60
select * from car where price between 40 and 60

7.离散查询
查询汽车价格在（10,20,30,40,50,60）中出现的汽车信息
select * from car where price=10 or price=20 or price=30 or price=40 or price=50 or price=60

select * from car where price in(10,20,30,40,50,60)
select * from car where price not in(10,20,30,40,50,60)

8.模糊查询（关键字查询）
查询car表里面名称包含奥迪的
select * from car where name like ‘%奥迪%‘ %任意n个字符
查询car中名称第二个字符为‘马’的汽车
select * from car where name like ‘_马%‘ _任意一个字符

9.排序查询
select * from car order by price asc asc升序（省略）
select * from car order by oil desc desc降序

先按照brand升序排，再按照price降序排
select * from car order by brand,price desc

10.去重查询
select distinct brand from car

11.分页查询
一页显示10条 当前是第3页
select * from chinastates limit 20,10

一页显示m条 当前是第n页
limit (n-1)*m，m

12.聚合函数（统计函数）
select count(areacode) from chinastates #查询数据总条数
select sum(price) from car #求和
select avg(price) from car #求平均
select max(price) from car #求最大值
select min(price) from car #求最小值

13.分组查询
查询汽车表中每个系列下有多少个汽车
select brand,count(*) from car group by brand

查询汽车表中卖的汽车数量大于3的系列
select brand from car group by brand having count(*)>3