查询数据：

简单查询：

1.查询所有数据：

select * from 表名

#select * from info 查询所有列

2.查询指定列：

select code,name from info

3.查询指定行(条件查询)：

select * from info where nation='n001'

select * from info where code='p001' or/and nation='n001'

4.给列制定名称

select code as '代号' name as '姓名' from info

5.模糊查询

select * from car where name like '%/_奥迪%'  #'%'---表示在‘奥迪’前后可以出现n个字符，'_'表示在‘奥迪’前后可以出现1个字符

6.排序查询

select * from car order by price #根据价格进行排序，默认升序(asc)，后面加desc降序

select * from car order by price asc，oil desc #先根据价格升序排列，在次前提下出现价格相同的项再按油耗降序排列

7.去重查询

select distinct brand from car

8.分页查询

select * from car limit 5,5 跳过前面5条取5条数据

9.统计查询(聚合函数)

数据条数：select count(主键/*) from car

取最大值/最小值：select max/min(price) from car

取平均值：select avg(price) from car

10.分组查询

select brand,count(*) from car group by brand

select brand from car group by brand having count(*)>=3

11.范围查询

select * from car where price>=40 and price <=60

select * from car where price between 40 and 60

12.离散查询

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

高级查询：

1.联合查询

select code,name from info

union

select code,name from nation

2.连接查询

形成笛卡尔积

select * from info,nation where info.nation=nation.code

为了查询效率更高可以筛选出重复的数据：

select info.name,info.sex,nation.name,info.birthday from info,nation where info.nation=nation.code

select info.name,info.sex,nation.name,info.birthday from info join nation on info.nation=nation.code

3.子查询

---子查询的结果作为父查询的条件使用

i.无关子查询

---子查询和父查询没有关系，子查询单独拿出来可以执行

a.查找民族为“汉族”的所有人员信息

select * from info where nation =(select code from nation where name='汉族')

b.查询所有厂商是“一汽大众”的车

select * from car where brand in(select brand_code from brand where prod_code in(select prod_code from productor where prod_code='一汽大众'))

ii.相关子查询

查询油耗低于该系列平均油耗的汽车信息

select * from car where oil

select avg(oil) from car where brand= 该系列

select * from car a where oil