CRUD操作（20161017）

上午：

(7)范围查询

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

select * from car where price between 40 and 60

(8)离散查询

select * from car where price=30 or price=40 or price=50 or price=60;

select * from car where price in(30,40,50,60)

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

(9)聚合函数（统计查询）

select count(*) from car

select count(code) from car #取所有的数据条数

select sum(price) from car #求价格总和

select avg(price) from car #求价格的平均值

select max(price) from car #求最大值

select min(price) from car #求最小值

(10)分页查询

select * from car limit 0,10  #分页查询，跳过几条数据(0)取几条(10)

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

select distinct brand from car

(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

分组之后，只能查询该列或聚合函数

取该系列价格平均值大于40的系列代号

select brand from car group by brand having avg(price)>40

取该系列油耗最大值大于8的系列代号

select brand from car group by brand having max(oil)>8

 下午：

高级查询：
1.连接查询
select * from Info,Nation
形成笛卡尔积
select * from Info,Nation where Info.nation=Nation.code

select Info.code,Info.name,Info.sex,Nation.name as '民族',Info.birthday from Info,Nation where Info.nation=Nation.code

select * from Info join Nation on Info.nation=Nation.code

2.联合查询
select code,name from Info
union
select code,name from Nation

3.子查询
子查询查询的结果作为父查询的条件

（1）无关子查询：子查询执行的时候和父查询没有关系
查民族为'汉族'的所有学生信息
select * from Info where nation=(select code from nation where name='汉族')

查询生产厂商为'一汽大众'的所有汽车信息
select * from car where brand=()
select brand_code from brand where prod_code=()
select prod_code from productor where prod_name='一汽大众'

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

（2）相关子查询
子查询在执行的时候需要用到父查询的内容

查询汽车表中，汽车油耗小于该系列平均油耗的所有汽车信息

select * from car where oil<(该系列平均油耗)
select avg(oil) from car where brand =(该系列)

select * from car a where oil<(select avg(oil) from car b where b.brand =a.brand)